The attached patch is from upstream bug #536531 and is rebased for gnome-control-center-2.28.1. It adds a "make default" button to the backgrounds tab of the appearance capplet. Useful if you don't like the default green background in gdm 2.26 and 2.28 or gnome-screensavers "lock screen" dialog. I believe Fedora and Ubuntu uses it and probably other distros as well. Thanks for considering.
Created attachment 210455 [details, diff] rebased patch
Committed in the main tree with a revbump, many thanks for the patch, and for reporting ;)