To make the duration of your photos match the duration of your music

  1. Adjust the timeline so that the full music track is visible.

  2. Select all of the photos in the photo track (see Selecting motion paths).

  3. Next grab the right-most point on the right-most photo and drag to stretch the overall duration of the photos. Drag it to match the duration of the audio. When there are many photos (150+), it can be difficult to grab the right-most photo. You might have to zoom the timeline in that case.

All of the durations (motion duration, hold durations, transitions durations) get stretched proportionally.