you need to first email them and ask that it be 
taken down. If they don't comply in a reasonable 
time (like division said, 7 days sounds right) 
then you can take legal action. Technically, it's 
theft of intellectual property, though most 
prosecutors don't like to take up such cases 
unless the thief is selling for a profit. 

As a last resort you could get the copyright 
holders involved, but be careful how you phrase 
your emails to them. Having scans of our own cels 
on our sites is just a hair shy of being illegal 
as well. They could try to come back at you. I 
recommend putting up a disclaimer on all your 
pages as to who actually owns the copyright to 
the image. That'll cover you somewhat if they try 
to come back at you. Also, if these image thieves 
try to tell the copyright holders that they 
didn't know the images were copyrighted (which 
they will), the rightsholders can point to your 
disclaimer as proof that they did know and posted 
the images anyway.

If you're feeling REALLY spiteful, you can email 
the thief's Webhosting service. They are legally 
obligated to investigate all allegations of 
criminal wrong doing and report the findings to 
the appropriate legal authorities.

Good luck.

KF 
(who's not a lawyer, but would like to play one 
on TV :)