Tough question here. In some cases, people are taking images and giving them new life in new contexts, and everyone wins. In other cases, people are ripping off photographers and artists who deserve to be paid for their work.
Not sure where to draw the line. Not sure we want to let lawyers decide.
Personally, I’m very shy on using photos. That’s why I have a lot of bookmarks for sites at which I can look up images and figure out whether they’re in the public domain or otherwise explicitly available for use. That distinguishes me from about 99% of Internet users.