To change the background image of a DVD menu:
Make sure the menu whose background you want to change is showing in the iDVD window.
Click the Media button to open the Media pane, and then click Photos so you have ready access to your iPhoto library.
You can also drag an image stored elsewhere on your computer using the methods described in step 3.
Use one of the following methods to add the photo to the menu background:
Drag a photo from the Photos pane to the menu background, outside any drop zones. When your pointer is over the menu and you see a green circle with a plus sign, you can drop the image.
Click the Drop Zone button (shown below) to open the drop zone editor. Drag an image from the Photos pane into the menu well in the drop zone editor.
With your pointer over the menu (and with no button or text object selected), press Command-I to open the Menu Info window. Drag an image from the Photos pane to the background well (the center well, shown below).