Hello - I've followed your directions in the manual for adding a background image:

1) create folder with app and .background folder with the background image in it
2) create read/write dmg via DropDMG from the folder
3) change dmg's view properties and set the background image (by dropping the image from the dmg to the view properties' background image drop area)
4) create read-only dmg from read/write dmg via DropDMG

I did all this on Snow Leopard and it shows up just fine there. On Leopard, however, the background doesn't show up and the window size is actually slightly off.

This is all with DropDMG 2.8.6.