An animated image combines multiple images in one file. The result is a animated gif, displayed sequentially, giving the appearance of movement.
GIF (Graphics Interchange Format) is one of the oldest image file types, introduced by CompuServe in 1987. It was very widespread on web as the image file type who can give the feeling of movement.
Almost every ad networks allow gifs usage, and in fact is one of the most popular file type used in advertising.
There are rules for the use of gifs in banners. For example, in Adwords, animation length can’t have more than 30 seconds. Animations should stop after 30 seconds. The animation velocity should be slower than 5 FPS.