I am on the prowl for the proper format and dimensions for adding custom images to the image pool. After poking around in the help document the answer does not seem to be readily apparent; although there is mention of keeping it under a specified file size. Looking for attributes similar to those assigned to the stock icons in the Appearance Pool.
I don't think it matters much provided the console can read the file type. For example I have tried .png .jpg .tiff .bmp and they all work.
.jp2 .heic .exr .gif .svg .tga did not work, interesting TGA did not work since the console seems to use .tga files for icons and such (I probably exported the wrong flavor). There are probably more valid types but that is all I checked.
as for the Size you could probably just take a screenshot of onPC or on the console and use that as a guide.
My guess is when you import the image into a pool the console makes a native version in locally that it actually uses.
Additionally, if you want to look at the stock appearance pool items you can go to:
~/MALightingTechnology/gma<version>/shared/resource/textures/ on mac
or C:\ProgramData\MALightingTechnology\gma<version>\shared\resource\textures\on windows and they are in there.