When Apple release iLife '06 it included a "feature" in iPhoto called photocasting (similar to the all successful podcasting). So basically I'm guessing it's just iPhoto subscribing to some rss feed right? I was initially interested in how this worked, so I've had a quick google search to see what sorts of things come up. Here the summary of a user that was posted on ones of Apple's mailing lists:
"To sum up, the "photocasting" feature centers around a single
undocumented extension element in a namespace that doesn't need to be
declared. iPhoto 6 doesn't understand the first thing about HTTP, the
first thing about XML, or the first thing about RSS. It ignores
features of HTTP that Netscape 4 supported in 1996, and mis-implements
features of XML that Microsoft got right in 1997. It ignores 95% of
RSS and Atom and gets most of the remaining 5% wrong.", http://lists.apple.com/archives/syndication-dev/2006/Jan/msg00020.html
A comment on that post also mentions that Flickr is able to output iPhoto rss streams. So my investigation ended when I got distracted and started browsing through the interesting photos links. Here's a sample of an interesting collection of crazy signs I came across:
Seems like its not a good day to be a stick person.
As for photocasting, I think I'll be leaving iPhoto alone on this one. I think just a simple blog producing standard rss has this area covered. The only convenience Apple has provided is the fact that it imports directly into iPhoto which means you can do other funky iPhoto things, but if you don't own a Mac, that's pretty useless too.