Someone correct me if I am wrong, but in my experience if you create a slideshow in iDVD OR using the slideshow funciton in iPhoto, it limits the slideshow to ONE song.
Now, you can use multiple songs the following ways:
1. You can edit the songs you want together using Garage Band and then output that newly created song and use it in iPhoto or iDVD, OR
2. Create the slideshow in iMovie. Pictures just drop in, as does music. You can then adjust which songs play where and when the stop and start. You can even set it to automatically insert transitions/fades between each photo and it uses the ken burns effect. (Under File>Project Properties). Then share the media, open up iDvd make your DVD and add the new slideshow VIDEO that you created and burn the dvd.