MD Softworks has a fabulous little utility called LeoColorBar which fixes the transparent menu “feature” in Leopard. It’s pretty simple, and simply puts a stripe (in a color of your choice) at the top of any picture you choose, and then sets that picture as your desktop background.
It doesn’t modify the original image, but rather stores a copy in your Pictures folder called “LeoColorBarDesktopPicture.jpg” with the appropriate crop settings, etc.