The strongest legal claim probably belongs to the photographer IF they copyrighted the picture. If so they own all legal rights and most definetly could stop the usage and maybe even collect some money from the offender.
You are not required to "copyright" anything that is an orignial creation of yours by means of government registering. ANYTIME you create something, IE: photograph, literary work of any type, it's automatically has a copyright and belongs to the person who made/created it.
And just because it's on the net doesn't not give anyone else the right to use it... it's still copyrighted material and must by law have the copywrite owners permission to use it. Looking at it or using it for personal use is sometimes implied and granted, but almost always, professional use is not and royalties are due to the copyright holder, unless the owner relinquishes his rights by giving the rights to public domain.... To ensure that you preserve and prove your copyrighte, the easiest thing to do is make a copy of your material (photorgraph, document, or whatever) and put it in an envelope and mail it to yourself. When you receive it back, the cancel date on the stamp will serve as a copyright date and will hold it's weight in a court of law.