It is good that magneto offers watermarking for securing the catalog/product images but this security is not sufficient for advance internet users. Suppose, following is the link for your watermarked image

    http://www.yourwebsite.com/media/catalog/product/cache/1/image/de684549e4151748e56ee0dd7902c19f/m/y/my-first-image.jpg

But if anyone removes the cache/1/image/de684549e4151748e56ee0dd7902c19f part of URL so that it becomes

    http://www.yourwebsite.com/media/catalog/product/m/y/my-first-image.jpg

He can view the image and copy it. To avoid your Magento catalog images fron getting stolen, put an .htaccess file in media/catalog/ folder in Magento with following code in it

Options +FollowSymLinks
RewriteEngine on
#Following line allows the actual images to be accessed by admin end directly
RewriteCond %{HTTP_REFERER} !^http://www.yourwebsite.com/.*$ [NC]
#Following line allows the watermarked images to be accessed directly. Rule says that if URL does not contain cache
RewriteCond %{REQUEST_URI} !(/cache/) [NC]
#This is the page where visitor will be redirected if tries to access images directly.
RewriteRule .(gif|jpg)$ http://www.yourwebsite.com/do-not-try-to-steal-images.html/ [R,L]

 Hope this helps.