From the wonderful world of wallpapering, here's an update from your local paper-hanger...
This message is especially for those who run Ubunutu 12.04 on systems that are not well endowed in the department of cpu cycles per second. (It's also likely to apply to 11.10). If you right-click on your main display, you may select the menu option "Change Desktop Background" and this provides a window for you to select your desired background wallpaper. Most choices of wallpaper are static and display a single image, while those thumb nail choices that have a small clock on them are a slide show. The default 12.04 slide show called, "Ubuntu 12.04 Community Wallpapers", is executed from... /usr/share/backgrounds/contest/precise.xml This xml file contains instructions to display the individual images, like this... <static> <duration>1795.0</duration> <file>/usr/share/backgrounds/Twilight_Frost_by_Phil_Jackson.jpg</file> </static> ...which will display one image for 30 minutes. The .xml file also contains instructions to transition from one image to the next, like this... <transition> <duration>5.0</duration> <from>/usr/share/backgrounds/Twilight_Frost_by_Phil_Jackson.jpg</from> <to>/usr/share/backgrounds/Precise_Pangolin_by_Vlad_Gerasimov.jpg</to> </transition> ...which will spend 5 seconds performing a dissolving from the first image to the second image. This transitioning process appears to require quite a lot of processing, to the extent that a slower cpu may appear to hang or be extremely slow in responding during this transition and for perhaps 30 seconds or more after the transition appears to have completed. Fix #1: Select a single image for the wallpaper background. Fix #2: Edit the .xml file and remove all the <transition> ... </transition> sections, so you switch between images rather than dissolve. Fix #3: Get a faster CPU. Wishing you all the best with your wallpapering, cheers, Ian. PS: As an example, I set up an xml file to be continuously transitioning over 5 seconds from one image to another and back again. This kept both my dual core P4 3GHz CPUs continuously busy at about 70%. The main processes executing being nautilus and compiz. If I just switch from one static image to another every 5 seconds, my CPU's are at less than 10% busy.