Changing the length of multiple stills simultaneously

1. Open 12-7.prproj.
2. Press the backslash key (\) to zoom the Timeline
3. Marquee-select (??) all the images
4. Right-click any of the selected images --> choose Speed/Duration --> change Duration to 10 seconds and click OK
Only the last images expand  to 10 seconds.
Try to change Duration to 10 frames and see the difference.
5. Press Ctrl+Z
6. Select all the images clips again
7. Right-click any of the selected images --> choose Speed/Duration --> change Duration to 10 seconds and select the Ripple Edit, Shifting Trailing Clips --> click OK
All clips expand to 10 seconds.