You also can't stop someone probing your code to find the image link; and if they're inclined to hotlink your images; they'll probably be even more inclined to do so if they encounter lax security.

Good idea though...

I'd advise looking into some server side techniques. Quite alot of the things you've been asking about here with Javascript (cookies, redirection, dynamically generated content, referer detection [which is what you'll need for this]) are much better suited to server-side scripts and technologies.