Hello darmatt,
If the picture is in iPhoto, then all you need to do is to find the photo in your library from the link on the side bar in the Libraries list. Once you found it, then drag it it to the timeline. Once that is done, look at the side bar again and you should see a tab for music. Find the song and drag it under the picture and it should fill in the amount of time the picture will play for. Take a look at the articles below for those steps with more detail if you need them. Let me know if this works or if you need further assistance.
Add photos and video clips from iPhoto or Aperture
http://help.apple.com/imovie/mac/10.0.6/#/mov865570171
Add audio and music
http://help.apple.com/imovie/mac/10.0.6/#/mov91a895a64
Regards,
-Norm G.