Previews and paid downloads for a stock photo shop

Your stock photo site sells licenses to use photos, at $15 to $60 each. About 3,000 photographers sell there, and the library holds 2 million photos. Photographers upload about 4,000 new originals a day, each 20 to 60 MB.

About 300,000 people browse the site each day, and each sees around 60 photos in search results and on photo pages. Anyone browsing sees only watermarked copies: small ones in search results, and up to 1,200 pixels on the long side on photo pages. About 5,000 licenses are sold a day. A buyer gets the original file and can download it again later from their account. Nobody who has not paid may get the original.

What files do you keep for each photo, where do they live, and what happens when a buyer presses Download?