The one way that I know of is to make an invisible folder in your disk image (an invisible folder/file is one that has a "." (dot) before the name, without the quotes), and take your pre-made image and copy it to that folder. You may need to do all of this in the terminal, since I don't think you can make invisible files from the Finder.
Open the window where you want your image to display. Hit Command-J ("Show View Options" in the "View" menu) to bring up the View Options window, and select the button at the bottom under "Background" that says "Picture" (make sure the window is in icon view, otherwise that option isn't available). Navigate to the invisible folder you made, and select the image from there. To get to the folder, you will probably need to type the path in directly (since it is invisible); hit Command-Shift-G to bring up the Go To dialog, where you can type in the path of the directory. If it is a disk image you have mounted on the desktop, the path will be something like "/Volumes/DiskImageName/.InvisibleFolder/image.png". If you're not sure, just drag the folder into a terminal window and it will give you the correct path.
I guess you could omit the invisible folder altogether and make the image itself invisible, but I think the invisible folder is a bit cleaner.