If you don't want people to use your images, don't publish them online. Anyone can grab a screenshot if they have a mind to. And owing to how the web works, there is no way to prevent images from being downloaded. That's the only way people can see images in their browser.
To discourage theft, use low-resolution images and watermark them heavily with your name and site's URL.
If you're serious about protecting your copyright, use a paid digital barcoding service like Digimarc. Your copyright information is embedded directly inside the image in a way that humans cannot detect. You can get the Digimarc plugin for Photoshop free from Adobe Exchange. But Digimarc's service has an annual fee.