Well, I haven't tried the app yet (someone else here may have though). I also don't know if there are websites that specifically produce SPB mobile shell backgrounds, though based on the product site, you can just look for wallpaper sites that feature images in the following sizes (depending on your Windows Mobile Device of course):
-480 x 800
-240 x 400
-480 x 640
-240 x 320
And of course, you can crop images down to those sizes.