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 slideshow.
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.
Click the Media button.
Click the Audio button.
This opens the Audio windowgiving you a view into iTunes (as well as a few other tools, which you'll use later).
Click iTunes in the top window.
From here, select the playlist you wish to access, or just leave it pointing at the library and find the song you want to add to your movie.
You can also use the Search field to find what you are looking for.
Drag the song to your timeline. iMovie will import the song to your file and place it wherever your playhead is parked. (Or you could place the song at the start of your movie by dragging it there.)
You could also move the playhead to the precise 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.