A situation has arisen today with the popular Duchess Kate blog, whose administrator is concerned that she may have to shut down her blog after posting some photos and screenshots that were challenged by the copyright holders. The seriousness of this situation underscores the need to be very, very careful in your image selection, use, acknowledgement, etc. Copyright is a legal issue and you don't want to get yourself into a sticky situation. [Read Duchess Kate's post about the incident.]
I have encountered this issue in my professional world, wherein a cease-and-desist letter and the subsequent removal of unauthorized copyrighted material is usually sufficient to resolve the problem. However, better to be safe than sorry.
Here are three quick guidelines and links from About.com to help you protect yourself from breaking copyright laws.
Follow @HRHDuchessKate on Twitter to participate in the ongoing conversation, or comment here with your views and advice.