Attach a video to a static image? Well, its not my area and i am a little bit scrappy with this.
So we have this little device , that we want to show how a real human hand stick to some different places .(for example door,window,gate ,etc)
Because we cant go physically over 50 locations, i was thinking about this 

I will picture one video, where a hand stick the device to some flat green/blue area, then replace that area with a different picture(s) .

1.Is it possible to make this look real (so a normal eye cant tell)?


Is there any better colour to use on the background ?


3.What is the simplest software that do that on a mac? Can iMovie do that ?

Thank you.
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.