Let's say you have a still image in iMovie, and you do the usual Ken Burns on it. The limitation of Ken Burns is you can only have 1 start and 1 end point. What if you want to focus on a few things? Want to start a new one where the old one left off? Continually zoom in on different areas in the image? It's hard to align the next clip where the first one left off. I faced this problem when, after my Ken Burns, I wanted to dim the image, except a certain area, to draw attention there.
Just copy the clip after setting the Ken Burns, paste it after the original and swap the Ken Burns start and end points. This way you can set a new end point. Or you can crop instead of Ken Burns, so the whole clip will be the end point, effectively freezing the final position.
I used the second method, exported an image to Photoshop, dimmed it, and returned it to iMovie.