I suggest you ask the webmaster to put a link to 
your site under the images, you know, a 
little 'thanks to littlepoop for these cel 
images' or something.  That's generally what I do 
when I catch people using my art/fiction (which 
technically is copywrited by me).  I used to have 
a 'pirates' section on my links page, but eh, I 
haven't had that up in a while, I just keep them 
in my bookmarks list to keep an eye on them.

Since you technically don't own the image, you 
really can't do much, especially since this 
person didn't get the images directly from your 
site.  You could ask them where they DID get the 
images, and then go after that site and do the 
same thing and the one before it, etc.  I mean, 
for all you know, your cel images are in 
someone's 'free pic' gallery somewhere, and I 
think if so, that's the person you should be 
concerned with.

And if you're looking for a good story, this one 
time, someone sent me a fanmail about a fic, and 
I asked where they read it, and they gave me this 
site that pirated aaround fifty stories... 
heehee... and I frightened the webmaster so bad 
she sent apology/request emails to all the other 
authors... But then, that's not a cel.  And that 
was with the aid of not-so-passive hacking.  
*cough* *looks innocent*