Did you provide the sitemap url to Google or do you put it into your robots.txt file? I have actually just extended the sitemap work of two users. I would suggest to remove the .html from Zenpage pages as they are not meant to have a suffix (Wordpress pages don't also) in opposite to the image links.
That was an insane something not considered. Bless you.
I provided sitemap urls to Google (three, actually: xml, rss, txt); last error, they suggested I DELETE robots.txt, and did.
Got it to partly working RSS that way, if that's a clue. Other two still fail, completely.
But if that ain't: I see my new options now around the roadblock. Bless you.
With Zenphoto 1.3 the sitemap url would be http://snarko.com/sitemap.php (as noted on the plugin documentation). That is the one you should submit. There is no xml, rss or txt url for a sitemap with Zenphoto, unless you make your own static one. OF course you should make sure that the robots.txt file is not blocking something.
Sadly SEO and Google is a kind of science (or should I say mystery..;-)) by itself sadly. In the end it comes all down to the content.
Anyway, you should really update as even the RSS feeds have been improved.