Google updated its image SEO best practices and Google Discover documentation to confirm that both schema.org markup and the og:image meta tag serve as sources when the company selects image thumbnails for Google Search and Discover results. The clarification, published on March 2, 2026, arrives after years of ambiguity over which metadata signal carried more weight - a question with direct implications for click-through rates and organic traffic.

Glenn Gabe, President of G-Squared Interactive LLC, flagged the change on LinkedIn within hours of it going live, summarizing the update for the SEO and marketing community. The update touches two separate documentation areas: the image SEO best practices guide and the Google Discover help pages - each receiving new sections that spell out, in concrete terms, how Google decides which image from a given page gets selected as the preview thumbnail.

What the documentation says

The core clarification sits in a new section added to Google's image SEO best practices document, titled "Specify a preferred image with metadata." According to the updated documentation, "Google's selection of an image preview is completely automated and takes into account a number of different sources to select which image on a given page is shown on Google (for example, a text result image or the preview image in Discover)."

That framing - "a number of different sources" - is significant. It confirms that no single metadata tag holds exclusive authority. Three specific methods are listed for influencing which image Google selects:

  • Specifying the schema.org primaryImageOfPage property with a URL or ImageObject
  • Specifying an image URL or ImageObject property and attaching it to the main entity using the schema.org mainEntity or mainEntityOfPage properties
  • Specifying the og:image meta tag

The primaryImageOfPage property, defined in schema.org version V29.4 (published December 8, 2025), is specifically documented as indicating the main image on a page and is used on WebPage type entities. The mainEntityOfPage property, its inverse being mainEntity, indicates a page for which a given thing is the primary subject described - a broader structural signal that Google now explicitly links to image selection. The mainEntity property, conversely, indicates the primary entity described in some page or other CreativeWork and accepts a Thing as its value.

These three schema.org properties now sit alongside the og:image tag as recognized inputs in Google's thumbnail selection process. This marks a shift from how many practitioners have historically thought about the two systems - schema.org and Open Graph - as serving entirely separate purposes (structured data for search, Open Graph for social sharing).

The Open Graph protocol was originally created at Facebook and is inspired by Dublin Core, link-rel canonical, Microformats, and RDFa. Its stated goal is to enable any web page to become a rich object in a social graph. The four required properties for every page under the protocol are: og:title, og:type, og:image, and og:url. The og:image property accepts a URL representing the object within the graph and supports structured sub-properties including og:image:width, og:image:height, og:image:type (MIME type), og:image:secure_url, and og:image:alt.

According to the Open Graph specification, an image tag such as og:image:width accepting a value of 400 and og:image:heightaccepting 300 would describe a 400 x 300 pixel image served over HTTP. Secure versions use og:image:secure_url to provide an alternate URL for HTTPS environments. The protocol also supports multiple images on a single page - when more than one og:image tag appears, the first from top to bottom is given preference during conflicts.

Google's move to formally document og:image as a thumbnail source for Search and Discover - not just social platforms - represents a meaningful convergence. The same tag that publishers have long managed for Facebook link previews and Twitter cards now directly influences how a page appears in Google Search results and the Discover feed.

Discover-specific image guidance

The Discover documentation received parallel updates. According to the revised Google Search Central page (last updated March 2, 2026), Google recommends images that are at least 1200 pixels wide, high resolution at a minimum of 300K, and formatted with a 16x9 aspect ratio for Discover eligibility. These specifications are not new requirements but their explicit inclusion alongside the metadata guidance reinforces the connection between technical image implementation and Discover performance.

The updated Discover documentation notes that "Google tries to automatically crop the image for use in Discover." If a publisher chooses to crop images manually, the guidance advises ensuring images are well-cropped and positioned for landscape usage, avoiding automatic aspect ratio application. For instance, cropping a vertical image into 16x9 means ensuring the important details land in the cropped version specified in the og:image meta tag. The max-image-preview:largesetting or the use of AMP is listed as a requirement for enabling large image previews.

Google Discover has become the dominant traffic source for news and media websites, accounting for two-thirds of Google referrals according to research published in August 2025 - a statistic that makes thumbnail optimization an increasingly material concern for publishers. The updated guidance lands at a moment when every percentage point of click-through rate carries amplified weight.

Content is automatically eligible to appear in Discover if it is indexed by Google and meets Discover's content policies. No special tags or structured data are required. Yet the practical reality - underscored by this documentation update - is that image metadata does influence which thumbnail Google selects, and a better thumbnail influences whether a user taps through from the feed.

Best practices outlined by Google

The updated documentation lists four overarching best practices when choosing images for schema.org markup or the og:image tag:

  • Choose an image that is relevant and representative of the page
  • Avoid generic images, for example a site logo, or images with text embedded in them
  • Avoid images with extreme aspect ratios, such as those that are too narrow or overly wide
  • Use high resolution images where possible

These recommendations apply equally to both schema.org markup and the og:image tag, treating the two as functionally interchangeable inputs to the same automated selection process. The explicit warning against logos and text-heavy images points to a long-standing gap between what publishers often deploy for quick implementation and what Google's systems prefer when constructing a visual preview.

Context: why metadata signals matter now

Google Search expands title link sources, adding OG title as a 9th option - a development reported in August 2024 that showed Open Graph metadata already influencing how Google displays page titles in search results. Today's image documentation update follows the same logic, extending the reach of Open Graph tags further into Google's core search presentation layer.

The timing is notable. Organic click-through rates for informational queries featuring AI Overviews dropped 61% since mid-2024, according to research published in November 2025 by Seer Interactive, which analyzed over 25 million organic impressions. In that environment, the visual thumbnail attached to a result carries more persuasive weight than it did when CTRs were higher. A compelling, accurately specified image can be the difference between a click and a scroll-past.

Samsung's One UI 7 update earlier in April 2025 defaulted devices away from Google Discover toward Samsung News, raising concerns about publisher visibility at the surface level. Against that backdrop, optimizing what appears when content does surface in Discover becomes more pressing. Publishers who already invested in Discover optimization now have clearer, documented guidance on the metadata levers available to them.

The updated documentation also arrives in the context of broader structured data changes. Google in June 2025 deprecated seven structured data types including Book Actions, Course Info, Claim Review, Estimated Salary, Learning Video, Special Announcement, and Vehicle Listing, describing the move as part of "simplification." Yet here, Google is adding clarity rather than removing it - distinguishing the image thumbnail guidance from the broader deprecation trend. Schema.org's primaryImageOfPage, mainEntity, and mainEntityOfPage properties remain active and are now explicitly tied to a visible, measurable outcome in search.

Google in December 2025 made updates to its December core update that caused severe Discover traffic disruptions for publishers - some reporting 70-85% declines before Christmas. The March 2026 image documentation update does not address ranking directly, but it does give publishers a more precise technical lever to optimize for the visual layer of both Search and Discover.

Technical implementation paths

For publishers and developers working to act on this guidance, three implementation paths now have explicit documentation backing.

The first is the primaryImageOfPage approach. Adding this schema.org property to WebPage structured data with a URL or ImageObject value points directly at the intended thumbnail. This is the most targeted signal for publishers using JSON-LD structured data on article or landing pages.

The second path uses mainEntity or mainEntityOfPage in combination with an image URL or ImageObject. These properties establish a relationship between the page (as a CreativeWork) and the primary thing it describes (a Thing). Attaching an image to that main entity is a contextual signal - telling Google not just that an image exists, but that it represents the core subject of the page.

The third is the familiar og:image tag. For many publishers, this is already in place for social sharing. The documentation now makes explicit that the same tag feeds Google's automated image selection for Search results and Discover thumbnails. Sub-properties such as og:image:width, og:image:height, and og:image:type can accompany the base tag to provide Google with fuller metadata about the image dimensions and format.

The three approaches are not mutually exclusive. Google states it draws from "a number of different sources" - meaning all three can coexist and collectively improve the probability that Google selects the preferred image.

What this means for the marketing community

For marketing and SEO teams, the practical read is straightforward: audit existing pages for conflicts between schema.org image markup and og:image tags. A page that specifies a logo in og:image for legacy social sharing reasons but a high-quality editorial image via primaryImageOfPage may behave unpredictably under Google's automated selection. The documentation's explicit warning against logos and text-heavy images applies to both channels simultaneously.

Publishers relying on Discover for traffic - a group that has grown substantially as AI Overviews have reshaped traditional search click behavior - now have documented confirmation that image metadata affects Discover thumbnails. Given the 1200px minimum width requirement, pages serving sub-optimal image sizes in their og:image or schema.org markup may be limiting their Discover performance without realizing it.

The schema.org V29.4 specification underlying these properties was published December 8, 2025, and the Discover documentation was last updated March 2, 2026 - both recent enough that many implementations predate the current guidance. A methodical review of structured data and Open Graph tags across high-traffic pages, using Google's Rich Results Test, is the immediate practical step available to publishers looking to align with the updated guidance.

Timeline

Summary

Who: Google, with the update flagged publicly by Glenn Gabe, President of G-Squared Interactive LLC, and covered by Search Engine Land. The guidance affects publishers, SEO professionals, and web developers globally.

What: Google updated its image SEO best practices and Google Discover documentation on March 2, 2026, to clarify that both schema.org markup - specifically the primaryImageOfPage, mainEntity, and mainEntityOfPage properties - and the og:image Open Graph meta tag are used as sources when Google's automated systems select image thumbnails in Search results and the Discover feed.

When: The documentation update was published on March 2, 2026. The schema.org properties referenced in the update were defined in V29.4, published December 8, 2025.

Where: The changes appear in two Google Search Central documentation pages: the image SEO best practices guide (which received a new section titled "Specify a preferred image with metadata") and the Google Discover help page. The Open Graph protocol specification is maintained at ogp.me.

Why: Google states the update provides clarification based on feedback, formalizing how publishers can influence which image Google selects as a thumbnail. The practical motivation for publishers is significant: Discover has become the dominant Google traffic source for news and media sites, images measurably affect click-through rates from both Search and Discover, and the relationship between schema.org and Open Graph signals in Google's image selection had not previously been documented in one place.

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