yep, show a low res image that's not worth printing or just don't give a damn. Or don't post online. Anything anywhere online can and will be copied and shared, no matter what you do. Sure, you can put some big watermark right through, basically ruining your image, or you can put a little on on the bottom, easy to crop out. But what you can see on your screen can be stolen and you can't do anything about it. If you want to show large, go for it. Unless you expect to sell large prints, does it really matter? Sure, it's irritating that some scumbag might copy/paste your image on their blog and pretend they took it, but really, so what. Nothing you can do about it, not even with a boatload of attorneys. As the music and movie industry had to learn.
Keep the resolution so that you can only make a small print (though even that can be up-rezzed to some extent) and let it go. Or don't post it.
Also note that places like Facebook basically can do what they want with your images until you AND everybody else that shared it removes it. Which is why I only post very small res images there, or samples but not the good ones I'll develop further.
It's the wild west online, and there's really nothing you can do, at all.