Wallpaper disappears
I'm running Mac OSX 10.7.2 on a Mac Mini (late 2011) Core i7. I have a dual monitor setup using the HDMI- and Thunderbolt output on my Mac Mini. But for some strange reason, the wallpaper on the right monitor keeps disappearing. It turns grey, but the monitor on the left had no problem remembering it has a wallpaper. The images for both monitors for use as a wallpaper are on the desktop.
First of all, why does it only affect a single monitor and second, how do I prevent is from removing the wallpaper?
Mac mini, Mac OS X (10.7.2)