It is down to the software developers, , a couple have made them, otherwise i have seen them being placed on reddit, but like the standard covers, not many people are making a standard size, they just seem to upload any size they want, as i think the dimensions shold be 600x900. But one drawback to the animated covers, or via the Steam Ui, is that it makes everything too busy on the screen. Image launchbox, just using video snaps all laid out together on the screen and all playing at once, you would end up going bonkers hahahahahahah