Question what are the rules about editing a submission after the deadline. 

I am not looking to fix any bugs with the game itself but I did forget to upload a cover photo to the games page

Itch.io pages can be changed after the submission date, so your good.

I just uploaded a new update for my Trijam submission, I think since the rating period is over this should be fine? I left the original submission still up for download though in case anyone still wants to play the old version.