This happened to me years ago, after someone "liberated" some below-par test prints of the band New Order from the waste bin of the darkroom I was using.  A few years later two of turned up in a record collector magazine, to illustrate a full-page article on the band.  I went to see a solicitor (what we call lawyers here in Olde Englande) who pointed out that in respect of the magazine publishing the article, the most I could hope for was to be paid the commercial rate for the two photos, whish was about £65 if I remember correctly.  I contacted them, provided them with copies of the negatives as proof of ownership, and they duly coughed up the £65.  I never went as far as tracking down who had supplied them and ultimately who it was who had misappropriated* them in the first place, If I'd had the time I would have.

*In English and Welsh criminal law, the owner of the discarded prints was and is still me.

My self-build system - Vista 64 on a Kingston 240GB SSD, Asus P5Q Pro MB, Quad 6600 CPU, 8 Gb Geil Black Dragon Ram, CoolerMaster HAF932 full tower chassis, EVGA Geforce GTX 750Ti Superclocked 2 Gb, Coolermaster V8 CPU aircooler, Enermax 600W Modular PSU, 240Gb SSD, 2Tb HDD storage, 28" LCD monitor, and more red LEDs than a grown man really needs.....I built it in 2008 and can't afford a new one, yet.....!

My Software - Poser Pro 2012, Photoshop, Bryce 6 and Borderlands......"Catch a  r--i---d-----e-----!"