Web pages with mixed ownership photos

[marciano]

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

[netmeg]

If you let your users upload pictures and aren't moderating them in some fashion, then that will always be a risk.

[marciano]

I have to trust they upload their own work, same as other sites. It is always a risk.
Anyway I let users to post comments on each image (apart of an existing contact form) then if somebody wants to denounce something we are able to perform some actions.

My concern is to let google know that I am the original page publisher (already done via G+ id) but containing images may not. Something like <img src... author='John Doe'...> no matter that John Doe has or has not a G+ account.