Chuck - if Yahoo syndication doesn't seem to care enough about "the validator" then... well... you know where I'm going with this, lol!
The validator validates pages that don't work and will spit out 50 errors for pages that are actually fine...
Here's the scoop - yes, I can SEE the pictures in a browser. I've tried several different PHP based rss feeds - which you'd think is a large purpose of using RSS to distribute your feeds in a popular format. NONE of them seem to work with the photopost image feed - all work with Yahoo, Nasa and the others. Here are examples:
RSS2HTML PHP service and server side downloadable:
YAHOO: Photo Highlight on Yahoo! News Photos
The feed into Wordpress - standard wordpress using the "SimplePie" plugin works perfectly with Yahoo but not with your validated but not working feed - and I love fish (I'm a big freshwater guy who may be using PP for his foray too but may be tempted to also just join the reef, lol.) Regardless of what happens in the validator, if you can't get the feed working with Wordpress as standard, it might make you wonder whether an adjustment should be made.
Perhaps here's a better request - could you suggest places that are using this feed and that it's working for them? I can't get it to just fill in what I need properly on the home page of a client site.