Steam store pages now natively support .gif animated images.
Previously, it appears developers managed to fudge animated images into store pages by disguising them as .png files. Valve recognised that customers appear to respond well to these, and has added official animation support.
“Over the past year we've noticed a lot of partners attempting to use animated GIF's in their store pages, specifically by renaming .gif files as .png files,” said Steam, in a brief update post.
“Customers seem to like these animations (in context of the store page) so this afternoon we rolled out an update that allows .gif files to be uploaded in the "Upload Custom Image" section of the store page.”
The post notes that these short animations can be a great way to provide a little showcase of gameplay. Support only extends as far as the store page itself, however - gifs will render as static images in the screenshot field and capsule art.
Valve also recommends developers take care to not overload pages with high-cost animated clips, and may remove them from pages with a large load size (15MB or above). Every second spent waiting for a page to load, after all, is a second a potential buyer may choose to move elsewhere.