Copyright can be weird. In some countries, it is illegal to take pictures of certain well-known buildings and then sell them as your own work, without written consent of the owners. This applies to famous buildings such as museums and the like. A shame, but true.
In some cases, websites that feature photography can be very generous with the rights they grant digital artists. For example, I know domai.com ("natural nudes" that won't offend anyone except the most purient) allows many of the images from certain photographers to be used as reference since many seeking to learn to sketch the human body don't have access to life drawing classes etc.
But the best course of action is to always seek permission, even if your final work is so heavily modified and "artified" that the original would become unrecognizable. I've often emailed a photographer with a heavily "artified" work where I used his/her photo as reference and ask if it's OK. In almost 100% of the cases they've been fine with it (most bot even recognizing their original work).
Always ask = always safe.
DaveyJJ --> artspin.com | widgetmonkeys.com | 729design.com
"Inside each one of us is an artist ... a child who has never lost the gift of looking at life with curiousity and wonder." Arthur Lismer