Picture in Picture

HI! I saw these awesome videos with blended models in real sets! How do you do this in Blender compositing?

To put a picture in a picture, just use an image input node, load an image and apply it into your render.

Here are some Tut links from the Wiki: http://wiki.blender.org/index.php/Tutorials/Links#Compositing

This is exactly what I’ve been looking for. Along with so many other things.
One more application to learn. Thanks a bunch!