A lot of websites don't like this. People link up their photos and eat up their bandwidth. Then they make a big deal sometimes about people "stealing" their photos and linking them without permission onto other websites (like this one ).
I think it's stupid. And by "it" I mean complaining about it. As someone who's a web designer I figure it's just a fact of the internet. There's nothing unethical about linking something you find in a public place (the internet) somewhere else as long as you don't take credit for it, aren't purposefully killing their bandwidth and it's not something like a Comic book which is out there for the money (stealing)!
A lot of websites try to protect from it and largely fail. I say stop trying. If it's online it's fair game.
I'm with Revo on this one. I take no credit for what's in the photo and it illustrates or underscores my point. It's like making a collage for a social studies class in high school. Besides they should be happy for the increased awareness of their material or product. At least that is the way I see it.