Merging images in MAC OS X?

[coolio2654]

How do you merge images in MAC OS X? Like, taking a pic, and placing it behind another picture. Like making a transparent background for a picture from another picture. Idk, like randomly putting Michel Jackson (he's awesome!) behind a window in a picture. Something like that. Plz tell me the program the the necessary steps to do it. Help would me much appreciated.

 

[VirtualTracy]

If you wanted to merge 2 images, do you have a link to them? Might be helpful if someone knows what the images are and if it'll work .... might be easier for someone to advise you then, if you don't have the app installed, what's the easiest way to do it and the simplest app available.