Bandwidth & Hotlinking

I offer a free file upload and storage server. I'm thinking of coding the script in Coldfusion, but need to know how to actually calculate the amount of bandwidth used for each individual image. This information will be available in the administrator page.

Also, I want to allow hotlinking, but I don't want people to link directly to the .jpg file. Instead, I want them to link to a CFM file and still be able to use that link in their <img> tags. Is there a way to achieve this?

Any advice would be appreciated, thanks.