I could be mis-interpreting what the new RSS features are, but I see the same errors on your site that I was experiencing on my own.
My 120gal Ver 3.0 - ReefTalk Gallery
In Firefox, PhotoPost displays an error that the picture cannot be found ...
In IE, I am presented with one of the built-in white error pages; Internet Explorer does not support feeds with DTDs.
However, if I turn off the spider-friendly URL feature in photopost, then the RSS feature works in either browser. I can PM you the URL to my development site with a working example.
Does this make sense?