One of the more popular methods of protecting the linking to your images is called "Hotlink Protection." All of our shared hosting accounts come with it as a part of cpanel. This prevents users from putting your images on their site and using up all your bandwidth. It does not however prevent them from downloading your images or copying them to their site.
In your /cpanel, at the bottom of the page, there is a button for "hotlink protection". This will allow you to enable it and not let anyone take your bandwidth.
By default, all outside sites will be unable to link to your images. You can also define sites that you WANT to have access to your images. Lets say you have a logo that you post in the about.com forums. You could add "about.com" to the list, and that website WILL be allowed access to your images.
If you want to be able to link your .gif banner wherever, but don't want people taking your .jpg files, you could protect just the .jpg files.
Also, please note that you will want to be sure to allow your own sites access to your images. Don't forget add-on domains, parked domains, subdomains, https, www, etc. If you find that one of your sites won't display images, and you have hotlink protection enabled, be sure to check your settings here.