Category Icon will display at android as icon at category or topics, good icon will help user to catch the information and easy understand about category. So make sure you pick the best icon for category.
1. Image ratio must 1:1,
because in android we strict width and height with same size, different ratio will make your image scratch, we recommendation you to use size Width 200px Height 200px
2. Image with single color (Monochrome),
this is a must because we use white TINT color when display category icon, Tint is transform color from image into into spesific color, and if your image not transparent backgroud the background will included to be color transform.
3. Transparent background,
Example image not transparent background when tint into blue color will be like this:
Here recommended icon source https://material.io/icons/