Go to iMovie/Preferences and set your Photo Placement to Ken Burns. Then
create a new project and import your still images into it. They should
alternately zoom in and out. Another way is to select a range of stills in the
timeline and click on the crop button and then set for Ken Burns. Click on the
blue arrow on the right to apply the changes. I believe that that will set the
in and out zoom for all of them. Then, again, you can manually set the zoom for
each individual shot by selecting it and clicking on the crop button, and then
making the zoom adjustment by arranging the adjustment rectangles in the
preview screen.
I don't know your specific situation but, generally, doing a range of
pictures zooming in and out can be a bit tedious and boring. I usually mix it
up with zooms from different angles, different durations, and an occasional
still shot. That's your call, of course.
-- Rich