Not without photographers permission. Some of my clients are photographers who have their works posted on other pages -- you should know that some photographers will be seek legal recourse, others may not. Best advice: contact the photographer and ask for permission to use it. It's not worth the risk -- the cost of defending yourself in a copyright claim could be thousands of dollars for one simple photo.
I do not want to fill you with dread over common day to day use of modern technology, as most people freely post work and they have no worries of others copying, reposting, etc. Most people are flattered by being tagged. Typically this is not an issue, but technically it could be. Your asking the question indicates there is more to this than is on the surface, and that requires a more cautious response.
Technically, the photographer owns the rights to his/her image AND she is informing everyone that she intends to enforce that right by having the watermark.
If she published (posted) the image to a public website that has a collective copyright license agreement which anyone posting must agree to, then the public can use any image posted there, as long as they obey the rules of the collective license agreement (often simply requiring identifying the source, author, photographer, when "sharing.") Tagging the owner is a nice gesture but does not automatically mean that you had permission to post the image. Read the license agreement of the website the photpgrapher posted his image onto.
Finally, if the image is of a "famous" person, or person whose reputation could be harmed by your use of their image, then there are other possible concerns.
For more accurate answers, we need more infrmation.
My disclaimer is simply that Avvo already has an adequate disclaimer.
By setting up a Facebook account the photographer agreed not only to Facebook's Term of Use agreement but also to its system of restricting access to and "sharing" the information that the photographer posts to his page. Unless the photographer restricts the distribution of his photographs via his privacy and application settings then I think anyone can lawfully "share" that content [see e.g., Facebook's Statement of Rights and Responsibilities at 2(4) ["When you publish content or information using the Public setting, it means that you are allowing everyone, including people off of Facebook, to access and use that information, and to associate it with you (i.e., your name and profile picture)" at http://www.facebook.com/legal/terms ]. Sharing information is the very purpose of social networking sites like Facebook. If a intellectual property owner does not want its photographs, videos or literary works shared by and among those who that person permits to view its Facebook page then that person can restrict such sharing. If not, then the assumption is that the material will be shared. Lawfully. Which is not to say that the material can be used for commercial purposes. I do agree, however, with my colleague that each fact pattern must be evaluated on its own. So before you share you should discuss the matter with your own copyright attorney.
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
Technically, the creator of the work, in this case the photographer, owns the intellectual property that flows from the work, specifically the copyright. Thus, you infringed the photographer's copyright, even if you gave credit where credit is due, and did not delete the watermark.
That said, the photographer may be interested in speaking with you about an alternative to the refund, in terms of a new session, or creating an agreement that addresses licenses for the photographer to use the work for their portfolio, and transferring the ownership of the copyright to you.