Best I can tell, the optimal sizes are 256x256 for the icon and 1320x240 for the banner. Iโve tried and failed to find a consistent answer on banner-sizing, but every answer has been different. Different apps also seem to display them differently, so I just picked a few that displayed well on different apps and averaged their sizes. ๐คท๐ปโโ๏ธ If you have a better suggestion, Iโm all ears.
Thanks for doing the work to test the best resolutions to use! If we happen to find out any other contradicting information about optimal sizes, I should be able to edit these as needed.
Here are a couple banners I made very quickly by basically just cropping down some wallpapers.
Banner 1:
Banner 2:
And here is a WEBP I was working on that I reworked just a bit to make it more suitable as a community icon. I also reduced the framerate/quality to get it down to just under 100kb while still being clear enough. I can always adjust the quality if necessary. Another neat feature is that the actual first frame is simply Liz smiling at the camera, so this should do well as a still image also. There is just some looping trickery to make it seem like when animated, it starts off with her turning her head toward the camera.
EDIT: Fixed banner 2's width to get rid of the tiny black bar on the right