Itss quite hard unless you're a good photoshopper.

In photoshop(/gimp):
Basically you need to take your Bart Simpson head and create an invisible layer over the head. From there you have two methods:

1) paint over whatever you want visible black, then place it over a clear-white layer and save the resulting greyscale image as barthead_alpha.jpg or something... name doesn't matter so long as you know its an alphachannel.

2) paint over whatever you want invisible white, then place it over a clear-black layer and save the resulting greyscale image as barthead_alpha.jpg or something... name doesn't matter so long as you know its an alphachannel.

Then apply that texture to your alpha of your NURBS material If done correctly (hoping you don't have any matte lines, and that I didn't get black and white reversed (which is an easy fix)) you'll have bart's head on a nurbs surface with everything outside his clear.