I found this great graphic that debunks the common assumptions about copyright. This is an issue I have had to deal with personally. I have had people blatantly steal my work and post it as their own, I have had companies use my images in marketing materials without permission, credit or compensation, I have had bloggers use my images without asking or giving credit or links back to my site and the list goes on, and on.
It seems that most people simply do not know the rules. Granted there are many images floating around the internet that are meant to be shared and used freely, but that doesn't mean that all images are the same. All of my images are protected under copyright, they may not be used without my permission, period! Educate yourself about copyrights and the rights of artists, and please, respect artists' work.