Attach a video to a static image?
The effect you are looking for is Chroma Key, and it is supported by iMovie (according to this Apple documentation).  The color green is typically used (and the technique known as Green Screen) because green is not the color of human skin, and thus easy to distinguish.  When green is a color that must be used in the scene (for example, if your subject is Kermit the Frog), then Blue Screen is used (and the technique is known as Blue Screen).  Sometimes no color works, in which case, people use bright white or dark black.  Chroma key software can work for those, too, but technically it's a Luma Key in that case.

As for making it look real--that's a skill.  But now that you know the name of the technique, you can find articles that explain how best to do it, and the pitfalls to avoid.