When you have a web site, it becomes quite common that some people feel so easy to just borrow your images that they do not even take the time to make a copy on their own web site. Not only do they use your artwork, but they also use the bandwidth you paid for.
Normally, there is a solution. You can modify slightly your website to ensure that if this happens, the image served is not the original one, but a modified one (either a big red X, or a message to the reader). But it is a little difficult to do by hand. A wbe site tool comes handy for this: HTMLbasix – Htaccess Disable Hotlinking Code Generator.