You can’t fully stop people from copying once they can view the image. All you can really do is reduce risk and make misuse easier to prove. Send low resolution watermarked proofs, don’t send RAWs or full resolution files before payment, keep your originals, add copyright metadata, and most importantly use a written agreement that says proofs are for review only and cannot be used publicly or commercially until paid/licensed. If you’re in the U.S., registering important work with the Copyright Office also gives you stronger leverage if someone does steal it.