Yes, go for Steam beta, go to the view with the big icons. Then, right click on whatever game you want to change and press "set custom image."
Wait, I'm in a beta?
I've been able to do this for months and I don't remember ever signing up for a beta
Also:
- Take screenshot as a DOF shot with $megaShotScaleFactor = 4; (was that even the value? whatever make it a megashot) then scale it down to help jagged edges
- Preferably also greenscreen over a screenshot of a nice build instead of just a white background; failing that at least a gradient so it's not a flat color will help
Also being as this is a creative thing related to Blockland and not just games in general, it should be in Creativity or General Discussion respectively (I'd go for Creativity)