You can try hiding the files using Storate Options and prevent them from being accesible by placing the normal images in a directory under the web root.
You would create a directory under your web root called photos or whatever you wish then in Admin => Storage Options you would place the normal url that exists and on the server path enter the new server path to the images and then hit save. The program will create all the appropriate directories and move the normal images. Mediums and thumbs though would still be linkable. Mod rewrite through htaccess is the only true secure way to prevent bandwidth stealing though.