I would still stick with iMovie - as it's probably going to be the easiest you'll find.
The process I would take is to first record the audio in a separate program. Audacity, QuickTime, whatever - just record it in a high quality format (16-bit, 48000hz, uncompressed, mono, AIFF/WAV, ideally. Record the audio in this app, whilst flicking through the photos in Preview or however else you view photos.
Once you've got the audio recorded, add it into a new iMovie project (should just be able to drag the audio file from the Finder into the iMovie timeline), then play through the iMovie project with your audio, pausing to add in the photos as per your narration, that you're now listening to. If your photos are in iPhoto, you can just use the media browser in iMovie to add in the photos directly from iPhoto.
At the end of that, you'll have a timeline in iMovie full of still photos, as well as your audio track. You can then add any transitions or effects (I probably wouldn't bother, but you may want to do the Ken Burns effect on your photos to get that zooming in/out effect, like, err, a Ken Burns documentary :p). iMovie has an export to YouTube function, so just use that to upload to YouTube. You can also export a higher quality version suitable for the Apple TV as well.