>I want to zoom in on the photos while the movie is playing, like in a documentary.
You should import photos with "Default Scale to Frame Size" set to off, and ideally have images with resolution at least equal to that of the video multiplied by your proposed zoom (scale) factor. For instance if you have NTSC, and you want to zoom in by a linear factor of two, images need to be twice 640x480 pixels or 1280x960. Don't make too many photos much bigger than this or Premiere Elements will struggle and perhaps crash.
Enter you photos and drag them to the appropriate places on the timeline
Right-click on a photo and select Properties.
In the Properties window select Motion.
Set the Current Time Indicator to the time you want to start zooming and click on the stop-watch symbol to set a keyframe. Set the scale and position as desired.
Move the CTI to the next (or last) position and set the scale and position as desired. Repeat if you want to zoom backwards and forwards.
Repeat for subsequent photos.