It always pains me to see this thread come up.  
Personally I've come to the conclusion that this 
action pushes certain peoples buttons.  
Personally I could care less, if someone steals 
my images I DON'T CARE!!!!  I own the cel, in the 
end thats all that matters (at least in my 
eyes).  

More to the point though, over the numerous times 
this comes up I've made the following 
observations and gathered the following 
information:

The only way that seems to prevent image theft is 
watermarking.  Though a very determined person 
(and someone with some know how) can remove the 
watermark with time.

We own the cels, but NOT the images.  The 
production companies own the rights to the images 
and can dictate how they are used.  This door 
swings both ways!  Just as someone steals images 
and uses them on their site, the site that they 
were stolen from can be seen as just as guilty 
because they never asked the company for 
permission to use the images on their website.  

Though you physically own the cel you still don't 
own the intellectual property or copyright.  The 
only 100% legit way of displaying cels that I 
have heard is for people selling them, they are 
allowed to make "copies" or scans of them for use 
to sell.

From people I know that have battled with people 
that have stolen images I am yet to hear of the 
cel owner winning.  Normally the offender is a  
total prick and uncooperative, or just flat out 
ignores the request.  If someone has had a 
positive outcome of this, I'd very much like to 
hear the story!  I think we all would, and how it 
was taken care of.