If people are 'hotlinking' to your image files, they are using your bandwidth which you will ultimately pay for. You can stop this from happening by placing a '.htaccess' file in the folder where your images are stored. This will only allow requests from your own pages to display the images.
Use the Htaccess Code Generator to create the code to place into your htaccess file to stop people hotlinking your images.
[ Htaccess Code Generator ]