Thumbnail is 512 × 512 and isn't covering the whole thing

Hey developers! I’ve been making thumbnails for my games recently and it says the picture needs to be 512 × 512 but when I make it that size it always has a tiny bit on the bottom of the picture is cut off so is there a way to fix this problem? Here’s the picture:

Thumbnail or game icon? If it’s the thumbnail I’m thinking of I wouldn’t do anything under1920x1080. And for a icon I would do 800x800.

Try making the size a little bigger, but not too big because it might compress it.