Linking directly to an image is a violation of copyrights and is called "hot linking." Here is an exceprt about directly linking to other web sites and the possible consequences. I've also included a link that gives further information.
Bandwidth theft or "hotlinking" is direct linking to a web site's files (images, video, etc.). An example would be using an <img> tag to display a JPEG image you found on someone else's web page so it will appear on your own site, eBay auction listing, weblog, forum message post, etc.
Hotlinking can have a lot of undesirable consequences. One is the so-called "switcheroo". If you've linked to an image on someone's server, what's to prevent them from changing the image you linked to? This can have humorous results. Since most sites, forums, etc. have strict policies about offensive images, it wouldn't take much for an aggravated webmaster you've been stealing bandwidth from to shut you down completely with an unwanted "switcheroo".
Displaying an image or file that doesn't belong to could be a violation of copyright, making you open to litigation. The owner of the file could utilize DMCA law to have your site shut down and your information given for use in legal proceedings.
further info : http://www.webweaver.nu/html-tips/hotlinking.shtml