They may have "asked permission", but they 
obviously had no respect for you or your property 
as they went ahead and stole your images before 
you even answered.  Unlike scans, cels involve a 
lot of time and money (well, my manga scans 
involve a lot of time, but not really money ^_^;;) 
and rampant reproduction of your cels could lead 
to forgeries or other trouble.  I would say "NO", 
flat-out (and demand that they delete all saved 
files from their hard drive and webpage), given 
their flagrant disregard for your property.  Had 
they actually waited for a response to post your 
pictures, then I probably would have allowed them 
to post one or two, MAYBE three, pics that I had 
edited with a visible and non-removable watermark 
(but, then, why would they want to post a pic like 
that? *LOL*) and demand a link back to your page 
and a clear disclaimer that they do not own the 
cel.

Personally, I wouldn't like this situation at all 
if I were you, and I would try to crack down on 
that sort of behavior (which is hard, I know)

~~ Sakura, who doesn't own pretty enough cels that 
people want to steal pics of them T_T