imovie 10.2.1: How to create photo clips of different durations

I want to create a slideshow with some photos of 2 second duration, and some series of photos at 0.5 second duration (to simulate movement). I tried in iPhoto slideshows, but couldn't make the program use various durations or accept a photo duration of under approx 2 seconds. So, switched to iMovie. I can set preferences to import photos with the duration I want (eg, 2 seconds), but when I change preferences to import the second set at 0.5 duration it changes all the photos to 0.5. Wondering if anyone knows how to import different sets of photos at different duration, or how to change the duration of a batch of photos- but not all the photos- once in iMovie. (I don't want to change the photos one at a time; there are too many.) Thanks a lot!