Not only me, but also many participants do not display the cover of the game. :(

Me too 😔

How can this be fixed?

Did you try to see if it's visible when using incognito mode? A friend of mine had the same issue and found out that the incognito mode didn't have the same problem. 

https://discord.gg/ENZEyRAs  Link to the server where you need to place a link to your game and its cover so that everyone can see it and appreciate it.

I'm having the same issue.  Over half the games don't have thumbnails.

Not an expert, but see if the cover image is hidden in the "edit theme" menu. Hope it helps!