Any idea how to change a game wallpaper?

Kinda stupid, but I like looking at PAYDAY: The Heist 's wallpapers when I click on that game in my library. At times, I feel the urge to take a screenshot of some epic moment/ random douchebag stuff. But, on screen-shotting, that particular shot shows up instead of the good ol' pictures depicting the crew in action. Any way to get these wallpapers back without backup/ removing my screen shots? Thanks!