I'm not an expert in this, so take it with a grain of salt. A photocast is the same thing essentially as a videocast since the formats are usually the same. What makes it a "cast" is that the content includes RSS enclosures that allow readers to subscribe to the content, like in the iTunes directory or Odeo, etc. This can be achieved by posting the content to a blog that has the proper RSS feed.
This page has some simple info regarding the process for creating video on a Mac. You can probably just replace the video creation process with how you create your slideshow and then process it in the format you want. The link above mentions .m4v, .mp4 or .mov video file enclosures.
Step 9 includes a link on how to add the content to the iTunes podcast directory.
Note though that iTunes doesn't provide the bandwidth or the hosting - just the feed, so if you get a lot of subscribers it may be expensive. Third party content hosting like libsyn provide affordable bandwidth for content.
Hope this makes sense, as I'm just kinda messing with it myself.
Other link that may help:
http://www.streamingmedia.com/article.asp?id=9314&c=33