Basically this is a simple URL change to make a picture that was small larger. Say you see a person who is not your friend so you don't have access to their facebook however you can see the tiny thumbnail of the picture that you would like to see in the full size. It's just a simple change of the URL and woo la you can see it larger.

example this would be a small thumbnail size pic:
http://profile.ak.facebook.com/vXXX/1234/16/qXXXXXXX_XXXX.jpg

simply change the "q" and you can get a better look. Here is how facebook breaks down their sizes:
Small="q"
Medium="s"
Large= "n"

so to see that picture larger you would change it to this:
profile.ak.facebook.com/vXXX/1234/16/nXXXXXXX_XXXX.jpg

OK so you're asking HOW do I get the URL for the picture. Well in Firefox simply right click on the picture and pic properties and you can see it there.