For now, here are some points to consider, but if they make absolutely no sense to you, I'll try to explain more if I manage to get the time to type up that guide I mentioned... 
Limiting colour count reduces file size considerably. Try indexing colours or using a posterize filter if needed.
Keeping frame count to a comfortably low level will help to reduce file size. Silky smooth animations are great, but reducing frame count will also give you more room for additional colours.
Since backgrounds are particularly tricky due to their canvas dimensions, really keep the above points in mind for them. I cannot tell you how many failed versions of my Breezy Meadow background I went through before finally getting it under 500 kb, and that was after using advanced layered APNG optimizations... Please don't suffer the same pain I did. LOL
Even if your files naturally fall below 500 kb, you can still use tinypng.com to reduce their file size even more before submitting them for review, which improves loading/display times.
In case you missed it, simple downloadable templates with correct canvas dimensions have been added to the help docs for each shop item, including basic stickers and sticker pack art. You'll find the appropriate pages listed under "requirements per item type" over here: https://gamejolt.com/help-docs/Shop/shop
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