I want to run a website like stock photos. How can I be sure that the uploader is real copyright holder of the picture? Is it possible to leave the responsibility of this copyright claim to the uploader or at last the webmaster is responsible for the website content?

It generally confuses me, as for example, stock photo websites needs form signed by the model for photos showing human face. How they can be sure that the signature actually belongs to the model? How they keep them safe from possible lawsuit in this case (e.g. if selling photos of a models with a fake signature?)