I want to put an image in my movie so that it will fill in the black edges on the sides. How do I do this?

I have a standard definition video I put into iMovie and set it to "Fit" so that it did not cutoff the top and bottom. Now I have black bars on each side. I am going to create an image, or two if need be, to cover those black edges. I cannot figure out how to insert those image and have it not picture in picture, greenscreen, or cutaway.