I have an exchange of services with a woman. She models for me and I take photos for her. Now she tells me that she wants to sell some of the photos and she asked me how does this work. We prefer to do it by the book so, if it is some regulation about photo ownership I would be glad if someone tells me where to find it it. If not, I guess we should share the benefits. If this were the case, does anybody knows what would be a fair percentage for each one? Thanks in advance.