The software I use to edit my photos allows me to add a watermark. It’s something you see more and more often: photos with a cringey signature of the photographer or just a cold name tag on them.
I heard of a photographer who started adding his personal watermark to his photos after an image of his was used in an advertisement without his permission. I get his anger.
However, such a watermark doesn’t stop anything. It is always on the least interesting part of the image, and thus the part that can be cropped away without pain. Furthermore, any good graphic designer is handy enough to make a watermark invisible in a few minutes.
Apart from that, watermarks are plain ugly.
What matters is conveying the message to prospective dirty thieves that they should keep their hands off your photos if they don’t want to pay. If you catch them using your work illegally, send them an invoice of 1,000 euros or more. They will not escape paying it. Every photo is copyrighted unless otherwise indicated.
They still don’t pay and you don’t feel like going to court? Copy the agency’s most successful campaign to promote your photography. Steal the slogans they used to make their best clients double their revenue. Just replace the names.
I know, it’s not exactly elegant to do so, but business is never elegant.
Other solution: make photos that are so special that the slickies of the advertising industry consider them inherently unsuitable to present to their clients. Transcend the Shutterstock garbage and you’re safe.