>Is there any way to prevent visitors to an HTML page from downloading the images which are on the page?
>
>Thank you

Not likely. If visitors see the image on the browser, it's on the cache already, not to mention that you can always do a screen capture.

You could put some code to disable the right-click menu, but you still have the menu options that you have little control over. Either that or the user simply disables the javascript.

Check out www.digimarc.com if you want to make it a little harder.