For startes, the only person they may have stolen 
the 'image' of a cel... is the very first person 
who got it from the studio.  HOWEVER, that is only 
the case if it was taken/sold illegally.  If the 
cel was sold like normal (by the studio or 
whomever had the legal option) then nobody stole 
the image as far as owning the actual cel goes.

Next, they say they didn't steal the image, but 
that is a complete lie and they must know it.  If 
by some random chance your cel was scanned for 
public use by the studio or you had a disclaimer 
on your gallery saying it could be used for so, 
then they have free use.  Other then that, which 
I'm sure you don't have, they are stealing, flat 
out.  It is YOUR image, that YOU scanned and put 
in YOUR gallery.  NO ONE else can use it, unless 
given permission.

They can feel free to use a screencap from the 
show of the exact same shot, or from the manga, or 
from almost any other source as long as, once 
again, the place they obtained it from allows them 
to do so.  They can even make the screencaps 
themselves or scan whatever they have themselves, 
and it is perfectly legal as long as they don't 
charge a fee to view them (or make any other kind 
of profit).  They are sprouting out bullcrap 
when they say that video and manga imagry is 
the same as cels/sketches.  See, if they charged 
to look at the images it would be a whole 
different issue, but they don't.  The items(
dvds,manga,cds,videos,plushies,etc.) were sold 
legally by the distributing company and can be 
used freely anyway anyone sees fit, as long as a 
profit is not involved.

They do not own the cel however, only one person 
does at any given time.  They have also already 
admitted that they took the image from you.  Since 
you own the cel, you are the only person that can 
use it's scanned image (except for profit).  
Others can use it to as long as you agree to the 
use.