Not sure if I'm misunderstanding you or you misunderstood me. I think it is important to list sources for photos. I just don't check to see if the photo is Public Domain or is under copyright.
It is possible. I had an acquaintance that won several lawsuits over copyright infringement.
What I'm trying to say is, the majority of people don't include sources for their pictures, so it's hardly seen as an important problem on steemit. Misinforming people, by posting facts without sources is where the issue is, in my eyes. If you have time to check the plagiarism tag, there have been some great discussions going on that raise allot of good points.