Ann Bens, thanks for the reply and info! The comments in the video you linked gave me another idea which I tried and it worked!
You can change the duration of all transitions using the below steps.
- Edit >> Preferences >> Timeline
- Set "Video Transition Default Duration" to the desired duration for all your transitions
- Select all the pics in the timeline
- Re-apply default transition to all (Shift+D)
Looks like this works because when you apply default transition, even if there already is a transition, it resets it to the current default duration. So you can change the default duration and reapply and then don't have to edit them individually.
Changing all picture durations and all transition durations is a common task if you're making a slideshow and trying to fit the pics to background music, for example and adjust the transitions to get the feel you want. When you have more than a few pics, doing them one by one isn't a great option. Fortunately this works.