Take a photo of your photo. Print it out, and take a photo of it, with the camera at an angle and the photo warped by perspective a bit. It won't stop people from stealing your work, but it does allow people to see you work, and doesn't give potential thieves much to work with.
Though to be honest, it's best to just accept that it'll happen or not put your stuff up online. You can try to do some occasional reverse image searches to see if anyone has stolen them and used them on their websites. But that doesn't prevent them from using them in print ads or on some social media sites like Instagram. You can also try to program your websites in stuff like Flash or Java (Flash isn't widely supported anymore, by the way). They have ways of allowing you to display images without letting them become directly downloadable. Thought that's a lot of work, and it's doesn't stop the screen grabbers.
I work for a print shop and we are ALWAYS getting companies coming to us trying to use stolen material. Some are actually quite large companies who should be able to easily afford paying for these images. When we tell them we can't print it (on the rare occasion we find out), they usually get angry and say something to the effect of "If it's on the internet, it's free for everyone", or "It doesn't matter. They'll never find out". Everything is about money these days, and no one cares about being honest, fair, or damaging their reputation. So until the law enforcement agencies, courts, and the law makers start to take this stuff seriously, it's just going to keep on happening, because the individuals and companies doing the stealing won't stop on their own.