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
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Adjust the timeline so that the full document is visible.
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Select the desired photos in the photo track (see Selecting motion paths).
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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.