Of course it doesn't give them the rights to the 
image, but lots of people would like to think it 
does.

Mostly, it's a matter of etiquette, which means, 
you can do whatever you want, but if you don't 
follow 'the rules' someone's going to get 
bitchy.  

-Some people object to you keeping a cel you 
don't own in your cel gallery' because it implies 
ownership, which is no longer yours.  I've seen 
several 'cels I used to have' or something along 
that lines gallery of cels that people 
traded/sold away, but its seperate from 
their 'current cel' gallery.  

-Some people don't want their cels reproduced, so 
they'd like to restrict access to all images of 
their cels.  Easy enough to understand.  I 
wouldn't give in though. I'd put a note about 
where its gone and do some image protecting, but 
that's it.

-Some people are flaming wackos.  You can't do 
much about them.  Just try to be nice.