Add or Share your GarageBand recording with iTunes. Once that recording is added to your iTunes Library, start iPhoto and go to your slideshow. Change the background music of your slideshow to your recording. Then go to Share -> iDVD. This will create a .mov file in your Movies folder and even start iDVD for you. You can start your iDVD project from there or close it without saving and start a new project. You can "Add Movie" in iDVD and select that particular .mov file.
Unfortunately, I am not in front of my Mac right now, but somewhere in iPhoto you get to select the format of your slideshow. You need to pick "4:3 TV". I think this comes up in the same menu you use to select whether you want background music.