It certainly is an interesting issue.  I can't 
speak from experience, because I don't have a 
gallery yet that anyone could steal FROM.  ;-)

I guess I'm more outraged when people steal 
original art that was designed specifically for a 
site (banner art, edited images, etc.).  Stuff 
that existed previously nowhere else but in the 
creator's mind and has only been displayed on that 
particular origin site.  That's copyright 
infringement.

With cels, we are dealing with art, true, but art 
that has already been displayed to millions of 
people commercially.  If anyone steals an image of 
your cel and claims to own it, that is totally 
wrong.  And at the very least, credit should be 
given to the original owner if an image is 
stolen/borrowed.  But consider this.  What if 
someone bought the original Mona Lisa---would they 
have the right to have all images of the painting 
removed world-wide???  I honestly don't know.

If no claim of ownership is made, I think the only 
issue here is common courtesy---something that is 
SORELY lacking across the breadth of the internet 
and which only seems to exist in pocket 
communities (like this one!).  For me, having the 
cel in my possession is enough.  If someone wants 
to display a mere digital copy, that is fine with 
me.  I still have the treasure in my hands.  ^_^

JP