Mac OS X Lion introduced Mission Control and a whole new way to organize desktops. In the process they made it possible to set a different image for each Desktop, but in the Dashboard we're stuck with a boring gray bathmat texture that can't be changed. But it turns that in Lion the new Dashboard is just another Desktop after all.
I wrote a random image fetching Dashboard widget (ImagesForever) that has a 'set desktop' button. 'Set Desktop' normally applies to the current desktop, but in Lion the Dashboard isn't an overlay, but a whole separate space of its own. So in Lion 'set desktop' now applies to the Dashboard background. A very cool and unexpected side-effect.
The widget's code just calls the usual AppleScript to set the desktop background, so it should be very easy to make a dashboard droplet that does the same thing. The AppleScript to set the desktop is quite simple:
tell application 'Finder' to set desktop picture to POSIX file '/path/to/MyImage.jpg'
[crarko adds: It's pretty well known that the Dashboard in Lion is just a Space, but this is a good application of existing knowledge.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20111210134819963