This is a perennial topic on many photography forums, as I suspect you already know.
The short answer is that if you can see an image on screen, you have already downloaded it. Even if somehow someone comes up with a weird way of avoiding that, all you would need to do is take a screenshot and you have the image. Bottom line is that it is pretty much pointless trying to stop someone getting the image at all.
That said, an image at the resolution for displaying on screen is, as you are aware, useless for any significant printing out, and all credit to Weebly that when you upload an image to a gallery at hi-res, what is presented in the browser is a suitably resized (and therefore much lower resolution) version. In short, all your viewers could steal from you is the Internet-suitable version, which would only make it of any significant use if they reused it on the web.
If you think someone has stolen an image from you and started using it elsewhere on the web, a service like Plaghunter could be useful since it periodically checks t'interweb to find matches for your photos and sends you reports of items found.
All I personally would do on your web site is to put a copyright notice somewhere to close the option of pleas of not being aware, and to put a bit of javascript into your html page to throw up a messagebox with a copyright message if someone tries right-clicking on the web page. These are easily foiled things, but no-one could say they were unaware your images are copyrighted, so any wrongdoing would be deliberate.
