So how do you prove an image is yours?
I did understand the ownerships problem but I also would point to the impossibility of hard proof with current technology.
As said, I put my energy in good clients and for my images that are used on personal websites without my permission I really don't bother.
Don't know where you live and what your country has in its law regarding copyright. I also don't know to what extend you want to go and how much money is involved with your problem.
Never had to do it myself but I heard of several cases in the Netherlands. If you are certain it is your image and it is used without your permission and without paying you there are normal ways to seek justice.
First call the person or company that betrayed you for info and explain, then put this in writing with the request of a reasonable fee. If no luck seek a lawyer for a first letter with warning for extra costs and fine.
If you want (depending on the law in your country but most western countries do respect copyright) you can get it to court. Be sure to have the original raw files at hand (the whole series makes it more believable) and eventually if needed or possible seek witnesses seeing you taking the pictures.
But it can be a long and hard road that is certainly not suitable for a small case. Then again, most of the cases have been decided in favor of the photographer, the company that first refused to pay left with the usual fee to pay including a fine (mostly the same or more as the fee) and having to pay for the costs of the court…
Also keep in mind this is not good advertisement for you as a photographer, especially if the fine is big the word will spread around in the business. It is always problematic in this cases, hence my advise to invest in good clients...