It's probably easiest to do the whole thing in GarageBand. You can import photos in just
the same way as a movie clip (go to Track -> Show Video Track and then you can drag and
drop photos that are in you iPhoto library), and then you can just change the length of
the region to set how long each image displays for. This can be edited to fit with your
narration that will be on an audio track.
GarageBand is full of little
surprises like that!
Hope that helps,