It has to be a static single image (160px by 144px) - can be an imagined game scene if you like, or just an image, design, artwork etc. The theme(s) are 1-bit - so just black and white pixels, no other colours allowed. And the secondary theme is "balance" - so something that comes to mind when you think "balance".
Read the restrictions in the rules about the amount of 8x8 tiles that can be allowed, as all artwork submissions will be created as a Game Boy ROM to play on real hardware. Use this tool - https://gb-studio-tile-count.netlify.app/ to check your image fits within the 192 tile limit.