I was using a .jpg image as my screen background. After UP8 finished installing, some of my directory icons wouldn't open, and when I tried to power-down and reboot, nothing happened (I remember seeing one message come up with rectangles where characters should have been). I finally was able to reboot by cycling power manually. Login went OK (I noticed a new login screen), but then when my desktop came up, the background was and still is just a solid light gray.
When I right-click on the background, a menu comes up with "Change Desktop Background" at the bottom. Clicked this, got a "Settings" window with my .jpg image in it, the legend "Background", a link to the main Settings menu, and - that's it. When I left-click on the .jpg image, it brings up the "Select Background" with the standard wallpaper images and a Wallpaper | Pictures | Colors | Flickr selector at the top. Selecting one of the wallpapers or one of the solid colors has no effect; the Pictures and Flickr selections just blank out the window. Nothing I've found so far will get rid of the light-gray background.
As for particulars, I'm running LMDE, 32 bit, with what I think is the Gnome classic desktop ("Applications" and "Places" menus at top left). Any helpful clues would be appreciated.