I uploaded a gif as a cover image today and it animated successfully, maybe its been fixed? :o
Viewing post in Gif's suddenly not animating
Cover images and banners are two different things- The cover image is the nearly-square image which is the "face" of your project when it's viewed as a store listing, while the banner image is the image shown at the top of your project page.
Cover images seem to work normally and when you hover the mouse over them they animate. Banners do not currently animate under any circumstances. Screenshots do not currently animate unless you click on them. Hope it can be fixed soon...