If I own a cel that has all the layers, and 
correct background (which is *very* rare with 
Dragonball cels) I would not say anything to 
people who display a screencap, or even a pic of 
the cel, on their site.  Since it has correct 
background there is no way to say for sure if it 
is your cel or a screencap, besides the fact that 
fansubs don't get that kind of resolution. 

However, I've basically given up on people 
stealing my cel scans. The only thing I do to try 
and prevent it is mark them with my site name. I 
used to be bothered by it, but now I just don't 
even bother to look. If someone actually takes 
the image and takes the time to alter it and edit 
out the site name - well, they have a lot more 
spare time on their hands then I do. :P  Not to 
mention the fact that it's aggravating to even 
attempt to ask them to remove it - they just 
don't understand the difference between a 
screencap, scan of a trading card, and a cel. 
It's all just a pic to them - heh, they aren't 
the ones who have spent hundreds of dollars on 
the cels.