On my website I store user pictures in a simple manner such as: \"image/user_1.jpg\".
I don\'t want visitors to be able to view images on my server just by trying us
You can look up "Hotlinking prevention" via htaccess and i think that should be a simple solution for the type of protection you need. However its not fool proof , people who will really want to get those images will find a work around by faking the referrer.
http://altlab.com/htaccess_tutorial.html