As this answer explains:

Ubuntu 12.04 Unity greeter has introduced selected user dynamic background, that means that when selecting a user from the available users list the background will change to reflect the selected user's desktop background.

However, I am puzzled, because in my experience this isn't consistently true. On one of my family members' computers, where there are three users, only one of the users' background shows in LightDM. None of the users are newly created. The user whose background is showing has a custom background; one of the users whose background doesn't show also does; and the other one who doesn't has the default switching wallpaper set. It is not a clean install but has been incrementally upgraded from about 8.04. Any idea what could be happening?

This is a similar question, though we're also having the opposite problem (default wallpapers not showing). (I do not have access to the computer mentioend above at the moment; when I do I will use some of the criteria given there to analyze the problem further.)