Hi Maxwell1295,
As I see it, unfortunately, it's not really a question of whose burden of proof it is to show ownership...
My first discussion would be with the guy I gave the picture to.
I would confront him first, asking if he had distributed the picture beyond his own personal use. The store owner doesn't know the origin of the photograph... Think in terms of a person who purchased a stolen TV at a pawn shop. They obtained the item from a seemingly legitimate source.... In your case, why should the shop owner you spoke to feel guilty?
Problem is, if you didn't make it clear you were providing him license strictly for his own personal use, you're already on a slippery slope.... Here's the reasoning: If someone gave me something for free, and it was without cost to duplicate that item, why would I assume that the copies had any value? The originals were free, why not the "free" copies?
Do you own the copyright? yeah. of course.
Did you act in good faith to protect said copyright? That question is far more debatable, unless you made it very clear verbally (and better in writing) that he couldn't give this picture out like candy.
I am NOT a lawyer, and this isn't legal advice, but I'll bet you'll struggle to "win" this one, in any kind of legal setting. I'd start with your first distribution point and pursue why they distributed your work beyond your intended scope.
And far more importantly, in the future, make sure you make clear the rights you're providing when giving away a "free" photo to someone else. This single step is your best defense against the problems you're currently facing.
I guess what I'm saying is that you should watermark or otherwise "protect" those things that you want protected, and not just hope for good will when dealing with "the public". Walmart (and most legitimate print shops) won't make a poster sized print of a picture with a copyright indication on it, unless you're willing to sign that you're the copyright holder. Since I started using Adobe CS3, I put copyright information into the exif of each DNG file as I copy it to my computer.
Would you leave your car unlocked with valuables inside on a busy street on Long Island?
Try making an insurance claim with no sign of forced entry. Even in your home. I've read many stories of refusal to pay claims where the property owner didn't take basic precautions to protect their property... Your photo and copyright is your intellectual property... So protect it!!!!
I get complaints on every other photo post I make here on Pentax Forums, because of my arguably distracting watermarks. Do I feel bad? NOPE! Failure to protect your own property is tantamount to saying it has no value.
My guess is that this one is a "lesson learned" and that's that. That said, I wish you luck in your pursuit. But be careful not to burn bridges where it's not a "clear cut case" of fault in the violation of your copyright, most especially because you work there, and that job probably pays for your photo equipment...
-Chris