sometimes, when I make modifications to the background image, nothing seems to happen until after I log out, and log in again. the recent addition of an 'apply' button made no difference, this has been there for a while. I haven't managed to pin down what the difference is between the times when it does work immediately, and times when it doesn't.
I'm duping this to bug 178505 which is a condensation point for recent background capplet breakage. *** This bug has been marked as a duplicate of 178505 ***