My students have discovered that although its not possible to change the background using the settings tool, it _is_ possible to set a background by right clicking on any image and choosing:actions/set as background kids are wonderful.
I did test the Kiosk Admin Tool to enable the users to change their Desktop settings. If you setup the debian-edu-pupils profile, under "Desktop Icons" the Desktop Settings are locked down. By removing this, users can change their desktop settings (including background). This worked fine by me.
Bjarne