Scrolling through submissions I see many games without cover images.

Top right on the edit game details page is the box for the cover image, above screenshots.

Even just uploading a screenshot as cover image can help.

As this is not part of your game itself I think it will be ok to add this now after submission if you missed it.

Thank you, just added a screenshot. Completely misssed that part, as this is my first Jam. Appreciated :)

It's good advice, definitely helps to increase the credibility of a build and shows that effort has been put into the project ...

I haven't done it yet myself, but it will be done shortly, wanted a day to relax before getting into the voting ...