Bug 1041710: Ensure that wallpaper images always exactly match the size of the screen

I always use self instead of bind() because it is right up at the top. With bind(), I don't know the meaning of this without looking ahead to the bottom of the nested function.  I see your point, and I'll make the change you ask for since this is code that you are more likely to maintain than I am, but note that I consciously choose to use self instead of bind().