If your project is HD dimensions, your photo should be 16:9 (e.g. 1920x1080)
If your project is standard def, it could be either 16:9 or 4:3 (e.g. 720x540)
You can do this within iMovie by using the Rotate, Crop, Ken Burs tool on the middle toolbar.
Use FIT to use the entire photo, but with black bars.
Use CROP to cut off the top and bottom, but what is left fills the frame.
I really appreciate the time you took to respond. And as my knowledge is apparantly less than basic, I hope you will help me a bit more. I hesitate to ask because I am sure it is a stupid question.
Does the 16:9 or 4:3 refer to the width and height ratio of the image? And does that mean, that no matter what size the image appears to be on the screen, if it meets the 16:9 or 4:3 ratio, it will show up properly (without the black borders) in iMovie?