There are a couple of things you can do, but the bottom line is that is it very difficult to prevent someone from actually getting a copy.

As Aaron2 says, a low resolution image is the safest bet, because then even if they copy it, they can't do much in the form of print or reutilize it for any purpose other than 'screen' display.

No matter what means you apply, unless they are visible on the photo at all times, someone could just do a low resolution 'print screen' capture, where they would keep what's displayed on their monitor on the clipboard, and then can be pasted onto a file, without really physically copying the original. The only thing you can do to 'remedy' this is to have a visible watermark on the image such as seen in the example below:

The real question is what is the nature of the images. Is it that you don't want others to  copy them, or are they confidential and private in nature. If that's the case, I would only display them on a computer where people cannot insert any removable media (jump drives, floppy disks, media cards, etc.) and no internet access so someone cannot take screen shot or a file and attach to an e-mail.

If you're looking to append a visible watermark, you could just open the image in a program like Adobe Photoshop and add a faded layer with text over the image. While it gets in the way of the actual picture, its your surest bet for people not reusing it. For a good tutorial on how to append a visible watermark, visit: http://rs.bahneman.com/protecting_your_images.php or search the web for 'protecting images'.

Good luck! Itay.