thanks for the review.
I don't know how to tell exactly but I'll try. First, go to your game page. For your game, it's this link I guess: https://cannibalisticcat.itch.io/sticky-dice. on the top left click on "edit game". And in the edit page on the top right there is "upload cover image". you can upload an image there. The recommended resolution is 630x500. And don't forget to click on "save".