I talked about this in my reply to Raphael (desoto_studio) on the thread “Do you paint from photos or screens.” In a nutshell: Most photos on Pinterest are copyrighted. If asked, sometimes photographers will allow their images to be used for painting reference. But if you use the art for profit (sale, promotion, reproduction, etc), and if a copyrighted photo can be identified as the source, it is not legal to use it as reference without permission.
The key question is: can someone looking at the art and reference side by side clearly see that any part of the art is a copy of the photo. If the answer is “yes,” and the art is used for profit, it is copyright infringement. Contrary to myth, there is no safe “percentage” that is legal to copy. Things like changing the sex of a figure, flopping the photo, etc. likewise are not sufficient change. Of course, with photos so accessible online, people do it anyway, but it isn’t legal. If you must paint from other people’s photos, you’re better off sticking to royalty-free images from the sites listed above.
You don’t have to take my word for it, the copyright question is addressed many places online. Here’s the “About” answer: http://painting.about.com/cs/artistscopyright/f/copyrightfaq5.htm