You can enable a Javascript that can catch the mouse right-click event and prevent the user from being able to right-click and download your images. However, any somewhat savvy user can easily bypass these scripts, and they mess with the overall usability of your website and therefore is not recommended. For a more detailed discussion of this topic see: How do I stop downloading.
The good news is that there are other ways to protect your pictures and make sure that you get suitable credit where credit is due.
1. Watermark the images:
This is the most effective way to ensure that another user cannot take your pictures and use them as their own - and what's better, if they do take the pictures and use them on their own website, they will effectively be advertising your work for you for free. If you are selling your work; what many people do is provide low-resolution watermarked images on the site, with an order form for people to request the larger unmarked images.
2. Publish your pictures under a license:
Creative Commons provides an easy way for you to choose a license for your work. Check out their license generator: Creative Commons.