How do I add customized wallpapers so they appear in the dynamic unity greeter?
LightDM only changes wallpapers if I have selected one of the default wallpapers. If I choose another image from my pictures, LightDM shows the default wallpaper.
Why is this happening, and how can I make LightDM show my wallpaper if it is not one of the defaults?
As @doug said in his answer, it works if you click on the little
+ symbol in the appearance settings: after you do that, LightDM changes wallpapers.
I bet you use an encrypted home folder - which is readable not before you log in. What you can do is to copy your background image for instance to /usr/share/backgrounds (as root, i.e.:
sudo cp ~/Pictures/backgroundimage.png /usr/share/backgrounds/) and set it as background image out of this un-encrypted folder.