There are a couple of things to think about 
here...I went through the other posts quickly 
(so sorry if it was mentioned and I missed it), 
and so far it seems like everyone is assuming 
that the cel pictures are posted in someone's 
*cel gallery*.  If the cel images were taken 
without the cel owner's permission and put in a 
big online *image gallery* (collection of 
screencaps, artbook images, etc), then that's 
another matter.   I, for one, would be a rather 
peeved to find my cels in a big image gallery 
like that if I hadn't given permission.  In a 
huge image bank, it's only a matter of time 
before the image crops up on fansites all over 
the web.  And, for many people, part of the 
reason for having a cel gallery is to share 
something that's unique and isn't on 50,000 
other sites.

Granted, if I bought the cel from someone else 
and the previous owner didn't care/gave 
permission for their scans to be used, then 
there probably wouldn't be a lot I could do.

As far as cel galleries go, I personally don't 
see too much trouble with a 'cel graveyard' type 
of thing, as long as it's clearly mentioned that 
the cels are no longer yours.  I don't have one 
myself, but I haven't sold many cels, either.  
If I did make one, I'd probably set it up as 
thumbnail only with a link to the current 
owner's site, if they have one.  That way, it 
would be clear that the cel is no longer mine, 
and a large scan would only be available if the 
new owner chose to provide one.

Pasiphae