I suggest contacting these people with proof first. Maybe they will remove your pictures.
The only certain method of internet protection is watermarking. That is a process of adding text that contains your identity(nickname, website, e-mail...) as a transparent layer to the picture. Basically you should put it on the middle or on the most important part of the picture, so that someone can't just cut it out and show the rest of the image.
If you're looking to sue someone, i suggest you e-mail every image to yourself before uploading them, when someone takes them, you will have proof of having these images before they did, which is useful in a copyright claim sue.