Why is Ubuntu continually changing wallpapers? Is there a way to stop it? The wallpaper section in the system configuration mentions no such option.
Try right-clicking on the desktop and choosing change desktop background and choose one that you like that does not appear in a stack. If it looks like a stack of pictures, then it's a slideshow and it'll keep changing.
Right-clicking on the desktop and choosing Change Desktop background will bring up the window that you see below. The image selected in the screenshot is a single image whereas the one below it(the one of scyscrapers) is a slideshow. Choosing an image that's not a slideshow will solve your problem