Prevent image file from being opened

I've noticed on some websites, if you get the direct link of an image and paste it into the browser location bar, it redirects to the homepage or a 404 page instead of showing the image.

Anyone know if this is easy to do or does it require a lot of scripting?

I'd assume it could be done via .htaccess somehow??

Thanks in advance.

Do you mean to prevent image hot-linking? Here is some information: http://www.alistapart.com/articles/hotlinking/