If you're fitting the photos to the length of your music, use fit photo to audio (see Fit photos to audio duration)

To stretch the duration of your photos

  1. Adjust the timeline so that the full document is visible.

  2. Select the desired 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 the desired duration. 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.