a lot of people do it,they get other peoples photos off googles image search,facebook etc,there is nothing to stop them so its easily done-consequences dont exist online until they get confronted for their stealing.
there is an easy way to stop photos appearing on google images,assuming the images are hosted on own webspace [which is easy enough to get free webspace]:
http://www.google.com/support/webmasters/bin/answer.py?answer=35308
if the images are being hosted on own webspace,so that the addresses are static based,for example:
http://www.website.com/images/chickens.jpg
there is a way to get back at these hotlinkers,used to do it when had once rented a server.
make an image large enough,to dreadfuly shame them-go as tame or as vile as ones morals usualy go.
save it as the same name,stick it under some other folder on the computer so it doesnt overwrite,and upload it to the same image folder as the one thats being hotlinked.
delete that one,but obviously make sure its saved elsewhere to be reuploaded when he quickly takes it off his site.
have found this work a lot better than simply telling people not to use images.
-its easy to find out if any image is being hotlinked by looking in the htaccess logs,assming thar be one.