I try to write a simple user script to enlarge the picture when you mouse-hover it :) ( like facebook fixer, but his code is somehow too difficult for me to understand ) I don't really know how to get the real image link of profile with the thumbnail :)
Real: http://profile.ak.fbcdn.net/v228/662/62/n1734060381_260.jpg
Thumbnail: http://profile.ak.fbcdn.net/hprofile-ak-sf2p/hs260.snc3/23246_1734060381_9960_q.jpg
You should use the Facebook API and get the user info I believe the field is called "pic_big".