Comment 87 for bug 931967

Ok, I've probably found a workaround:
Edit the file /etc/gdm/PostSession (which is executed at logout) and add the following line before the line "exit 0":
gsettings set org.gnome.desktop.background picture-uri file:///Path_to_your_desktop_background.jpg
The standard backgrounds are found in /usr/share/backgrounds/ if you need to know where to start looking.
This seems to solve the problem for me, and displays a black screen during the previously mosaic-garbled seconds of the login. But I wouldn't call it a "fix", though. And the privacy/security issue should still be looked into. Where is this information stored?