I've changed my desktop environment to Cinnamon on 32 bit 15.04, the problem is that I can't seem to change my desktop background by going to
System Settings->Background as I have selected a new background (several different ones I've tried to make sure it isn't just a bug affecting just one wallpaper) but no changes have occurred. I have also tried restarting Cinnamon (by the panel applet) in case the changes only occur after it is restarted and I did see the wallpaper I picked out while Cinnamon was restarting but once the restart had completed and I opened nautilus it went back to the old wallpaper I had before.
I rebooted my PC after attempting MiHa's answer and my background was the one I had wanted and then two system error messages appeared, leaving me with this desktop:
Notice how my desktop icons are doubled up. My unwanted background before I rebooted was:
and the one I wanted was: