but it all starts with photoshop's "layers"
Load one pic (original to be 'chopped").
Now load the "replacement" head shot or whatever.
Ctrl-a, ctrl-c to select all, and copy to clipboard.
click on the original, and ctrl-v to paste the pic into a second layer over the first.
You'll need to do a scale or a free transform on the new image to get it to match the one you're replacing...it gets a bit complicated to explain, but the method is simple.
It's get very detailed after that part, as there's several ways to do what you want to do.
I'll suggest these excellent tutorials over at
http://www.worth1000.comhttp://www.worth1000.com/tutorial.asp?sid=160978 Arsidubu's excellent face swapping tutorial.
http://www.worth1000.com/tutorial.asp?sid=161081 His mini tutorial on proportions during face swaps. (What to avoid.)
http://www.worth1000.com/tutorial.asp?sid=161090 How to use layer masks. (This is the *right* way to do face swaps, as opposed to using the eraser tool.
http://www.worth1000.com/tutorial.asp Full listing of Worth1000 tutorials...well worth digging into if you enjoy chopping images.
Also check out the DU Graphics group here...we have some very talented folks here who can provide insight and answer questions as well.
Hope this helps!
on edit: shoulda ran the spell checker.