Where does OS X store your current desktop picture? I read somewhere else that it was in /Library/Caches/com.apple.Desktop. But I don't have that directory.
Starting in Lion, I don't think Lion caches desktop pictures anymore (possibly because you can have multiple at a time, for different desktops/spaces).
In that case, use
The Apple delivered desktop images are stored in /Library/Desktop Pictures , so you can eliminate those (or seek those if you used an Apple picture for one or more spaces). Before Lion, the preference file that controls desktop images is text format any to find out the current image, open