I'm using theme Windows Classic to get rid of the aero effect (for the sake of speed for my developing works) but I enjoy having the background images/photos from the other themes (e.g. Bing Best) but I don't know how to do this after trying a while in the desktop personalizing settings of Windows 7. Please help if you know how to do it!
Though you switched to Window Classic Theme, you still can use Windows Theme's background image.
Personalization > Desktop Background > Choose Windows Desktop Backgrounds from Picture Location drop down. you'll see the background images of the Windows Theme you'd installed .
for your case, the downloaded themes are installed under
and if you choose multiple images, these will change automatically according to the time you've set. like automatic window background changer.