How do I increase the amount of wallpapers that appear on the menu for customizing the desktop with my backgrounds in 11.10?

I added more images to the respective folder in /usr/share/background as root, but the list does not remain persistent.

The images in 11:10 changes alone. So I want to add funds to enhance the environment. When I copy the images to the folder /usr/share/backgrounds the list is updated and increased. But only for the first time. Thereafter, the system ignores the other images, and the list of funds has become original as first.

Any help?