This is actually an interesting problem, because it's already solved but most people aren't using the solution: If you have a large file do distribute to a large number of people without authentication, use BitTorrent. As far as I can see there are two primary impediments to this:
A) Most browsers can't by default download large files P2P. You can actually write a BitTorrent client in javascript using Web Sockets if you really want to, but that's just horrible. What would be really nice is to be able to just e.g. embed a video into a webpage using a magnet link. There is no technical reason why this couldn't be implemented and rightly should be for large files.
B) Images are exactly the wrong size. They're big enough that you can't just ignore hotlinking but not big enough that you want to pay the overhead of connecting to 50 different peers instead of one to get a good transfer rate. But that just requires some adjustments to the protocol; if you're looking for realtime retrieval for display in a webpage you would probably want to use UDP and then use erasure coding to deal with slow/broken peers and packet loss. If you have a 60KB image, you can send a ~50 byte packet to each of a dozen peers and have ten of them each send 6KB (approximately four packets) to the target with 6KB worth of erasure bits from each of the others (which also allows the image to be constructed once 60KB of data is received in total from any collection of peers), and now the image is costing you ~600 bytes instead of 60KB. And if the image hasn't been received in 150ms, add more peers.
