Doesn't look like there is any way to do it in iMovie itself. All photos are forced to fit the movie's screen aspect, and there is no letter-boxing (boarding for media that does not fit).
I did manage to simulate what you want, or perhaps even better. I used the Pic Collage app to free style arrange some photos, keeping them smaller and centered top to bottom, then I had to rotate the result in Photos (because Pic Collage only created portrait collages). When importing to iMovie it looked like this.
That's a lot of work to go through for each photo(s), but you do get a bit more say in the results.
Most likely there is another video or slideshow app out there that will give you better options.
I just downloaded Adobe Premier Clip, and if all you want a simple way to combine photos and video clips into a short movie, this seems to work pretty well. There is a 'slight' Ken Burns effect, but the photos are presented entire, with letter boxing as needed. I only tried the Auto mode, so you might be able to kill the Ken Burns effect in Free Style mode.
The main caveat is that it doesn't seem to be playing well with iCloud Photo Library. I got around this by going to Photos first and viewing all the photos I wanted to add, only then did the older photos show up. It also adds a few extra frames at the end showing the Adobe Premier Clip splash screen. It's easy enough to edit out in the Photos app; though there might be legal(ish) issues if you edited it out and shared the video publicly.
It also crashed on me, once, while in the middle of downloading media. However the project was automatically saved and I lost nothing.
It's free, but you will have to sign up for a free Adobe ID. Can't get past the splash screen without one.
Hold in mind most of these pIctures were square. They were not cut off. There are a few portrait and landscape pics for contrast.
