It’s been long requested of Steam users to be able to set custom backgrounds for non-steam games and shortcuts, but Valve and Steam have kind of ignored the issue for quite some time.
However I just noticed that with the latest Steam update, which adds the screenshot functionality, it IS possible!
Because of this Steam has just become my favorite game launcher, it has everything I need now.
As you can see in the picture below, I have Fallout New Vegas added as a shortcut (Non-Steam game), yet there’s a custom background image present.
It’s as simple as loading up the game, and using F12 to take a screenshot. If you have more than one screenshot saved locally for a game it will randomly select one each time you select the game.
Unfortunately, these images do NOT carry over to the list or grid views, but perhaps a Steam-savvy user can figure out how Steam is using the background images and force them to be used as the “title cards” in grid and list views as well. I’ll take a peek into it, but if you don’t see this post updated then I haven’t figured it out.