We have updated the Game Cartridge requirement ; there are actually two different Game Cartridge designs:

• One is a 630x500 pixels image and must be used as "cover image" on itch.io
• The other one may be used as "poster" sprite in your microStudio project, whenever you also publish your game on microStudio.dev ; publishing on microStudio.dev is not a requirement, but it is cool if you can do it too

Check https://itch.io/jam/microstudio-jam-3 for all the official information ; do not hesitate to ask for help in this community or on the Discord server.