1. Establish Ownership of the Original Photograph: Look for the original photo in your archives and note the EXIF data – most people don't have the means (or the know-how) to modify this information. In case they are able to strip the data off the image, you can also use other photos in the same series to prove your ownership. If your images are online, Google and other searches can help you establish when certain images were uploaded.
2. Document the Offense:** Take screen caps, print them, then sign and date them. You probably have to get other parties involved – let them take their own documentation. Think of them as witnesses to the crime – their involvement gives your claim more credibility.
3. Stay Professional:** As tempting as it is to come in with your guns blazing and threatening legal action, most people who "steal" your photos are simply ignorant. If you ask them nicely to remove the photos, credit you for the images, or compensate you for your work, most people will comply.