You can't crop the video. You can't really crop the picture, but you can do a few other things.
Once the picture is in the time line select its clip. The larger display window will now have three buttons; Start, Stop, and Done. These are used to set the Ken Burns effects. Tap Start if it is not grayed out. Zoom (pinch gesture) and drag the picture in the window until it looks the way you want it to at the beginning of the clip. Tap End and set the way you want it to look at the end of the clip. Once it is set the way you want tap Done.
Play the clip and see it move from one state to the other. You can of course try (it's tricky) to use the same start and end positions for a still picture.
The picture can not be shrunk further than the screen screen size.