You can't. Even if you use HTML or JavaScript code to block any method of downloading the images through the browser's interface, someone could take a screenshot of the browser window while the picture is displayed.
In order to block someone from downloading the images through the browser, you could set the web server to tell the browser not to cache the images, and then check whether the
referer is a page which links to the graphic. Some people may have blocked their browser from sending a referer, and this method won't work if someone takes a screenshot as mentioned above.
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