Security for a Small Stock Photo Company

ExTex asks: "I am a commercial photographer, and I also run a very small stock photography company. Most of the sales that I make are to existing clients or from referrals. Currently, when I make a sale I upload a ZIP file of the image and create a unique web page for the customer to download. I pull the page shortly after the client has confirmed receipt. This is easy, when I'm in the office, but can be a challenge if I'm out in the field on a shoot. At some point I'd like to be able to have 400 of my best images already pre-zipped and loaded to my web host for quick download. I'm wondering how best to secure the images to prevent unauthorized download but also make it relatively easy for the typical un-savvy client."