I found many articles that were similar to mine, but none with my specific problem.
I have a dual screen setup and I am using Ubuntu 11.10- I changed my wallpaper from one dual screen image (1600X600) to another (2560X1024) using the System Settings > Appearance > Wallpapers menu.
It worked fine. Then (being an idiot) I decided I wanted to try some different ones that Ubuntu provided.
I chose one that came packaged (The Grass Ain't Greener 2000X1250) then I decided to go back to the original. This is where the problem started. Using the Wallpaper menu, I could no longer select any images that have previously been used as wallpapers. What Gives?