This one's pretty interesting, i wanted to change the user-icon in ubuntu 9.10 login screen but found that from this edition of ubuntu users were not allowed to mess up with it , so i searched and searched and found that there was a way out!
open the terminal , type
and you will see a window pop up, here you can choose the icon of your choice!
You can even add new icons in the folder /usr/share/pixmaps/faces and can choose from them
Happy Linuxing :)