How to put a picture on a video ?

[Tampon]

I have a video downloaded off youtube and I want to stick a pic of a mate over the top of the guy talking for a joke. I have no idea how to do this, any help ? ( I only have micrsoft editor on the xp ) so any software would need to be a free download. Oh his head moves slightly throughout the video so I presume I will have to place the picture scene by scene.

Cheers