Hi,
I have found with iPhone iMovie that the pinch function and the magnifying button work only on video clips, not photos. There is no "fit" button. A free video creative product that has a "fit" setting and will properly display vertical portrait photos and videos is iMovie for Mac. iMovie for Mac is a totally different app than iMovie for iPhone.
On your iPhone iMovie try putting your photo into a new project. Then tap the project in the projects screen and share out the project as a video by tapping on the share button (the square with the arrow in it at the bottom of the screen). The options presented will be "Save or Share Video", which you should tap, and that will give you the option to "Save Video". Tap that. The photo will save as a video in the iPhone Photos app. In iMovie, import it into your project as a video. (You may first need to crop the photo-video in the Photos app into the original vertical aspect ratio.)
After importing into iMovie, you should have a vertical photo-video in your timeline with the "+" and pinch functions available. iMovie still seems to want to impart a slight Ken Burns effect to it, so you may need to trim the clip in the timeline to the place in the video that shows the least zoom. You can control the display duration by adjusting the speed with the speed controls. Faster speed = shorter display time; Slower speed = longer display time.
Not an optimum solution, but might work. What you are essentially doing is creating a one photo slide show video. I think you could create such a slide show using several photos that you can then edit once importing into iMovie.
-- Rich
