Hi, yes you can add screenshots :)
After the jam, you are allowed to:
~ add screenshots / game banner artwork, edit the game's page theme, and edit the game's description on itch
~ fix game-breaking bugs (e.g if an entire level does not load, or something stops the game from being able to be completed) without changing anything else in the game
~ port the game to other platforms (e.g making a web build, or adding mobile controls) again without changing anything else in the game
Hope this helps and thanks for joining the jam!
Theo