You said the key word yourself. "sites".
They are not linking to our sites, they are 
linking to the image file itself.  Following the 
links, people do not see our pages, only the cel 
image.
I would have NO problem with this if they either 
a) linked to my gallery page and not directly to 
the image file.
b) (and I know a lot of the other people don't 
feel this way, but I've always been pretty 
carefree about what happens to my cel scans) 
saved the thing on their pages so MY bandwith 
wasn't getting eaten every time someone clicked 
on a link to THEIR site, without ever seeing MY 
site!
Most people like having their -pages- available 
on search engines, heck if we didn't want people 
to see the images we wouldn't have them on the 
web at all to begin with.  But if we wanted them 
up there as just a random image, and not as part 
of our personal cel gallery, then we wouldn't 
bother to build pages at all, we'd just put the 
images into an index folder and call that our 
gallery.