For Jennifer, it's no problem that her still photos have no intrinsic sounds. They're just photographs, after all. But Christopher wants to add music to his dynamic slide show.
To add music to your video, you never need to leave iMovie. Just as you accessed your iTunes library and playlists from iPhoto, you can do the same from iMovie.
You could also precisely move the playhead to the spot where you want the music to begin, and click the Place at Playhead button, which will automatically insert the music at the right location.
And that's it! You'll see the long purple track of audio underneath all your video clips, likely extending long past the end of your video if the video is shorter than the music. You can slide the song around to begin at any point you choose (although starting at the beginning is always a good idea).
And most important, now that the music is in place, you can make adjustments to the durations of your clips to better fit with musical cues in the song.