Then in the Terms of Service/Rules you say you don't allow them and monitor the uploaded images carefully. Can we please get back to the main topic now?
You could do this with an htaccess rewrite rule. Then resize the image with JavaScript. Yes, this is quite straightforward. Save this as ".htaccess": RewriteEngine On RewriteCond %{HTTP_REFERER} !^http://(.+\.)?mysite\.com/ [NC] RewriteCond %{HTTP_REFERER} !^$ RewriteRule ([^/]*\.jpg)$ /files/thumbnails/$1 Code (markup): Obviously change "mysite" to whatever your site is and change /files/thumbnails to whatever your thumbnails directory is. Drop the .htaccess file in your full size images directory.