ok, this is a little trick I read about on the web
Display Properties > Desktop > Customize Desktop > Web tab
Select New and browse to the image location > Click OK
Now drag the image over to the second desktop and from the image's pull down menu in the upper left-hand corner select Cover Desktop.
Right click > Arrange By and "Lock web items on desktop".
Now you should be able to select a different wallpaper for the primary monitor.