A few months ago we got a huge blip in activity level because a very busy site had placed a link to an image here on their home page. I changed the image's name. I could easily add the access directives to prevent other sites from linking to images here, but except when it noticeably affects our bandwidth I actually want them to be able to link to our images.
I take the same free and easy approach to images at other sites, including copying them here so I never lose them. So if at some point no one ever hears from me again, you'll know what happened!
--Percy