You can secure FTP through SSL certificates on top of windows user authentication (which I can advise because you don't want your password sent over the wire in plain text).
Downloading through IIS as a webserver would work just the way you download any file through your browser. I'm not sure how this works in FF for example but IE doesn't have a download manager to resume downloads, but you can ofcourse install a small (free) program called Free Download Manager which can resume file transfers from any source (HTTP, FTP, etc).
But are you sure you want to transfer 13 GB image files over the internet? That's going to take a while to transfer on a regular DSL or cable connection?