I'm not as familiar with iMovie for iPhone. On my iPhone 14 Pro it appears that clips that are not 16:9 aspect ratio will be cropped to fit iMovie's 16:9 screen. I don't see any way to fix that. Selecting the clip in the timeline and using the pinch method on the preview screen to shrink the clip to display the whole clip, does not work. As soon as I remove my fingers, the clip springs back to its original crop.
I think that the only solution for the future may be to record the clip in 16:9 aspect ratio so that it fits the 16:9 screen, and then there will be no cropping. Or possibly you can use the Ken Burns feature to pan over the entire photo. Or maybe you can place the photo on a 16:9 black blackground (or photo), and then it would fit the 16:9 screen, although with black borders.
-- Rich