Here is a handful of games using it:
- Mingle
- this game about chickens
- Lookouts
- Wayfarer
- Assessment Examination
- Vampire Survivors
- Tenjutsu
- Dynacore
In every case I've looked at, the game creator has uploaded an image that is slightly bigger than the dimensions for the frame containing their game (i.e. the game_frame div).
It also looks like you can use a GIF if you want to use an animated image; the chicken game and Assessment Examination linked above both use gifs.
Can you send me a screenshot of the specific issue you're having? I'll can see if I can figure out the problem.