The Edit Menu/Preferences/General preference for Still Image Default Duration applies only to images that are imported after the setting has been made.
To change Still Image Duration once the still is on the Timeline, right click the Timeline still, select Time Stretch, and set the still duration for that still in the duration field of the Time Stretch dialog. You can do this route for one file or more than one file at a time. For more than one at a time, select (all at one time) stills involved, click anywhere in the selection, select Time Stretch, and set the still duration in the Time Stretch dialog which will apply to all in the selection.
To select more than one file at a time
a. As you hold down Shift key, click each clip intended to be in the group selection
b. With the mouse cursor, draw a rectangle around the clips to be involved in order to select them
Please let us know if any of the above works for you.
I discovered that if I had already imported an image, then imported it again, the old settings was used.
Only images that had never yet been imported respected the new settings.
Thanks for the follow up and the good news that the Still Image Default Duration details are sorted out for the before and after import situation. Great follow through.