Oh, and why deep linking is NOTHING like book references
When you give someone a book reference, even with page and paragraph numbers, they still have to go out and find the actual book, so the author still gets credit and a royalty (even if it's in a library -- the book was purchased at some point). On the web, this would be like saying "if you want to see a cool picture, go to this guy's website, on this specific page URL". Nobody has any problem with this sort of thing, because you're still going to the other guy's site to see his content. He's properly attributed, he sells ads, everyone's happy.
When you deep link, you access only the content (like an image) itself, not the context, the author attribution, the web page it's on, or anything else. It's more like copying a passage from a book, putting it in your OWN book, and not mentioning where it came from. On the web, this is what happens when people link directly to an image on another site, showing the image in their site.
The worst case of this is in forum posts, where people sometimes deep-link to someone's photo as an avatar image. Because forum posts are viewed thousands to millions of times, and the avatar can appear hundreds of times in a single thread, bandwidth costs for the avatar can become totally INSANE.
It's not innocuous, and it's not cool. Don't deep link! It's bandwidth theft! Not to mention practically plagiarism...