Have you ever caught someone hot-linking an image on your site? If you don’t know what hot-linking is: (from Wiki):
Hot-linking is when someone uses a link to an image that is saved on another website instead of saving a copy of the image on the website that the picture will be shown on.

Why hot-linking is bad? Because when someone is hot-linking your image, he/she is using your server’s bandwith to serve the image to their readers. So let’s say you took a picture in Paris on a holiday and you put it up on your blog (let’s say the file size is around 1 Mb). I saw the photo on your blog and then hot-link it on my own blog. That means when my readers view the photo, they actually use your server’s bandwith instead of mine to display the picture. If I have 1000 readers daily looking at the picture, that means I’ve stolen 1000 * 1 Mb of bandwith from you and you are not even aware of it.

This post will describe how to use htaccess to prevent people from stealing your images & bandwith!