Yesterday I followed this answer on "How do I add customized wallpapers so they appear in the dynamic unity greeter?" to make my wallpaper appear in the login screen.
But instead of only changing the rights of the pictures I added to the /usr/share/backgrounds folder I also changed the rights of the pictures that were already in it (the standard Ubuntu 13.04 wallpapers) (I did this by using gksudo nautilus and then selecting all the pictures, right click --> properties --> permissions tab --> read and right for me, read only for group and others ) Now I have no wallpaper at all on the login screen (it's just purple with the white dots) and in the guest session the wallpapers are missing too.
I suppose the problem lies somewhere in the read and write rights I changed, but I don't know what's wrong with them.
Does anyone know how to solve this?