Bob,
Is it possible that you have locked the background layer but it is still the active layer? If that's the case it won't let you draw anything while that layer is still the active one.
You do want to have that background layer locked so you can't accidentally select the background picture (and then move it by mistake), but you'll need to change the active layer to start drawing other stuff. You do this by selecting one of the other tabs at the bottom of the window.