I've noticed recently that a few art sites I belong to seem to have some contributors who either don't understand copyright issues, or understand them and don't care if they are infringing on them or not, at least not as long as they can get away with it. I'm not saying all people do this - the vast majority try to be careful of copyright issues, since nobody wants to be hit with a lawsuit.
Another thing I've had to contend with is the fact that copyright laws vary from country to country, and dealing with sites that are located in a country where you don't live can cause some confusion. Generally, I try to work within the copyright laws of Canada, as well as the sites where I submit, because they work within the copyright laws for their own countries. For the most part, they meld fairly well - at least the sites where I participate, though there have been some differences I've noted.
One thing I cannot do is offer legal advice pertaining to copyright - I am not a lawyer and not well enough acquainted with every section within the copyright laws posted at the various agencies. When you encounter those "gray" or "fuzzy" areas, what you need to do, and what I would do when it's necessary, is find yourself a good lawyer (we have one) who will help to unravel the mystery for you.
Copyright law is not something to ignore just because you may not understand it, or may not think it applies to you.
Besides copyright, there are other issues to take note of, like "trademarks" which are different from copyright.
Here's just a few of the helpful links to copyright information I've got stored in my files.
US Copyright Office
Ten Copyright Myths
Keyt Law - Copyrights
Fair Use of Copyrights
Protecting and Prosecuting Your Images
Canadian Copyright Law
Copyright Board of Canada
Copyright Law in Canada
The Canadian Copyright Licensing Agency
Australian Copyright Law
Australian Copyright Council's Online Information Centre
The copyright Licensing Agency
UK Copyright Service
Design and Artists Copyright Society
Federation Against Copyright Theft
World Intellectual Property Organization (WIPO)