According to the announcement we can use gif, webm, m4v, and mp4 file formats. Y ou can try using .webm for your files, that would support proper transparency and the file size would be less than a .gif and have better quality as well. The downside is that it's not something you can easily do with Photoshop or similar programs, so you'll need to look into other software like ffmpeg to encode your files. It's not overly difficult as there are some guides on using ffmpeg, but it will probably take some of your time to test settings to achieve something you like that sits between good quality and small size. If you don't need transparency, stick with mp4 as that will surely be easier. If you will only be making pog-style animated tokens, a .gif should suffice as the only transparency is outside the token and has 0% opacity (so there's no need for the advanced transparency of the .webm). You can do both in Photoshop; don't know about GIMP. Hope that helps!
