Using a photo as background for green screen video in iMovie

I have a photo, and a video with a green screen background. I want the video (of me) with the photo as background. I've done what I thought I've read, but I'm missing something. This is iMovie on Mac.

• Pulled the photo into a project
• Lengthened the photo to the length of the video
• Pulled the video in
• Selected Green/Blue Screen from the dropdown.

Instead of just myself superimposed on the photo, I get what looks like picture-in-picture -- a picture of me with the green background showing, placed in the middle of the photo with the photo showing on the sides.

Any help appreciated!