really laid into them with your proof, and 
DEFINITELY threaten them with a harassment suit 
(even if you never follow through, they deserve to 
feel badly about this!).  Make sure you keep hard 
copies of all your emails to and from one another, 
your scans of the cel as well as their scans, and 
contact ebay about it as well.  

Care to share a link to their gallery? *evil grin* 
 No, reeeaaallyyy...I just want to look. ~_^