Adding images with voice-over in Prezi is a little tricky. Prezi suggests changing file formats to insert sound into your presentation (see here).

But here's one way that worked for me:

In GarageBand,

(1) Make a Podcast

(2) Share Podcast with iTunes (Share>Send Podcast to iTunes)

(*NOTE* You could also record voice-overs in iMovie, although you'll have less control over your recording than in GargaeBand.  In iMovie, simply click on microphone icon & record!)

In iMovie:

(3) New Project>Name & Choose Background>Create

(4) Drag & Drop image(s) from iPhoto  (from camera icon on right side of screen)

(5) Drag & Drop Podcast from iTunes into background (from music icon on right side of screen)

(6) Share iMove with media browser (Share>Media Browser)

In Prezi

(7) Insert "Image"

(8) Select your iMovie file (usually found under Media>Movies)

Additionally, screenshots made in Quicktime also work in Prezi.

To make a screenshot:

In Quicktime:

(1) File>New Screen Recording

(2) Hit Record button (it will record audio & computer screen)

(3) Hit Stop button to end recording

In Prezi:

(7) Insert "Image"

(8) Select your Quicktime screenshot file (usually found under Media>Movies)