There's a tool called 'HotLink Protection' in cPanel which does exactly what this thread has been started for.
The description follows:
HotLink protection prevents other websites from directly linking to files (as specified below) on your website. Other sites will still be able to link to any file type that you don't specify below (ie. html files). An example of hotlinking would be using a <img> tag to display an image from your site from somewhere else on the net. The end result is that the other site is stealing your bandwidth. You should ensure that all sites that you wish to allow direct links from are in the list below. This system attempts add all sites it knows you own to the list, however you may need to add others.
Very useful - I use it all the time to protect my images.
If your host doesn't have cPanel, tough luck mate. Oh wait, you could always change hosts.