Hello, I was googling the same issue.
I added 500+ foto is in project and started to edit about 200 and then decided that I wanted to change from 3 to 2 seconds per foto.
I am not sure if something changed but I am on VideoPad 8.75 and I am able to:
select one picture (double click), hold shift and select another picture further down the line in storyboard mode and change the time. All selected pictures will be changed! OMG