Alan,
I would suggest to offer an FTP upload. Since you do have a website and a domain belonging to it, it should be an easy thing to also activate the FTP port. Almost all Internet hosting providers should offer that. Then you can let your customers define their own directory where they can upload their files and send you an email that they did so.
That's the way one of my two stock agencies is handling the file upload thing. It's very easy and works fine. Also, this is the way I am administrating my own website.
Thomas