When I save images from Firefox, they can't be used as background images!

Hi! I have a small problem with Firefox that bothers me. I run on a Mac OS X Snow Leopard. When I see an image that I like, I right click and choose "Save Image". I store it in my Pictures folder. When I go to the manager that helps me choose a background, the picture isn't available to use! But when I save a picture in Safari, it is there for me to choose. Why does this happen? Thank you! == This happened == Every time Firefox opened