On use of others' images.
Mostly folks who make pictures of all sorts are happy to let others use them as it's both flattering and can bring in traffic. Usually there will be an exception for commercial sites, as we generally aren't happy with other people making money from our work.
Here's a guide for dealing with artists that create original art:
1) Ask permission to use copyrighted work. Usually you know where it's coming from and can find the author. And usually we're more than happy to allow use with credit.
2) Assume all art work is copyrighted. Original film, drawings or notes are pretty good evidence if it comes to a lawsuit. Copyright holders can notify the ISPs of infringement and under law, the ISP is obligated to shut down the site even before infringement is proved. This is a huge hassle for everyone. But I have done it when my work was stolen for a commercial porn site once.
3) If the author says no, go look for something else. There are images all over the place and someone's probably made something better anyway. And that artist is probably a nicer person.
4) If you can't contact the artist, the first choice would be a link to the place where the picture came from. The second choice would be a prominent credit very near the picture, or a website URL so the artist is getting some traffic for the work.
There are a lot of costs that go into making the art that we show. In the case of a recent specific picture that was used without permission, I traveled from Maryland to Pennsylvania and hired a far too expensive hotel room. Plus film and associated photo costs.
I sell my photography, but don't mind lending small scans out to folks. I do resent having it snatched without any compensation or even a credit and most photographers I know feel about the same way.