I have a photo website. 15% of the photos are from approved registered users. They agree my terms about uploading their images in my web pages. I include a photographer credit on right bottom corner. About identifying the site with google, every page contains a google+ button to MY google+ page It also contains <link href="https://plus.google.com/nnnnnnnnnn/" rel="publisher" /> I need some advice in order to respect google rules about my pages containing other photographers images not to be penalized because of possible duplicated or interpreted as stolen content. My concern is also about adding G+ links (to MY photo page) and Google publisher id would harm my site rank because of pages containing third-party photos. Thank you
You have a good reason to worry about that. But it also common in the website many photos are used for illeagal reasons, and the authors always know nothing about it. Check whether these photos have been protected, and you can contact the authors.
You can try posting a disclaimer on your website about that matter, so if Google penalizes you, there's a way you can protect yourself. Also try to contact the authors as lonking said.