Well, what Little Snitch is telling me here is that Photo Stream is actually streaming the photos out to Amazon’s S3 service. In the screenshot you see that /Applications/iPhoto.app/Contents/Library/ LoginItems/PhotoStreamAgent.app/Contents/ MacOS/PhotoStreamAgent is attempting to make an https connection to someplace.amazonaws.com (18.104.22.168). With North-American IP addresses, you can look them up at https://whois.arin.net and you can see that IP address is owned by Amazon.
While it’s a little surprising, it makes perfect sense that Apple would use the low-cost commoditized services from Amazon to host the Photo Stream data. We (Nerds On Site) certainly use Amazon’s array of services ourselves and for our clients, so this is a good thing!