Adding an image above a video in imovie
In my experience (2 years), there's not a great way to accomplish this. Here's how I would do it:

1) Make an additional "background image", in the dimensions of your typical video (16x9, I assume).

2) Drag your original image over that background image, at the top, leaving room for (later) the video clip at the bottom.

3) Save that image.

4) Bring that image into your iMovie timeline.

5) Drag your video clip onto it as a picture-in-picture.

:)