In terms of electronic screens (computers, cell phones, PDAs, tablets) what would be more accurate to say: wallpaper or background?
It depends what your intentions are. The 'wallpaper' is an image displayed in the background.
If you are pointing out that a context-sensitive menu may be opened by right-clicking on the background then the presence of a wallpaper image in the background is immaterial.
If you are pointing out that I found a great site with lots of wallpaper images to choose from, 'wallpaper' might be preferable.
Bottom line: wallpaper refers to the image, background refers to a concept in a multi-windowed environment.
Technically the more apt term would be background-image, rather than background. Considering the two, the difference could be that wallpaper refers to the background-image of the desktop, the window with the lowest z-order in a multi-windowed environment, while a background-image refers to the background of any particular window.