Special effects :)

Hi… i would like to ask a question but I can’t explain it in a clear way… so I’ll use an example.
So… for example, in this image (that is taken from a video):

http://www.realillusions.net/temp.jpg

there is a robot between the background and the boy… how can I do that? I don’t know how to get information from internet about that arg… can you help me?
Furthermore, I think that, in this video, the robot was added “directly” in the video, and that wasn’t added the robot over the background and then, at the end, the boy…

what you are looking for is called “rotoscoping”
it involves painting a “matte” or “keyhole” for the elements that you want to have “over” or in front of other added elements
then the whole thing has to be “composited”

if you do a google search on any of the “words” above you should find some info

That image was taken from the Icarus Tutorial:
http://boards.theforce.net/Fan_Films/b10015/17604478/p1

This Tutorial deals with putting a hand over a 3d object,
http://www.weirdhat.com/blender/compositing2/
one more:
http://www.blenderwars.com/tut.php?module=blendcg

Furthermore, I think that, in this video, the robot was added “directly” in the video,

Also take a look into this tutorial it deals with putting together 3d objects and video background.
http://www.blenderwars.com/tut.php?module=blendersaber