A valid RSS feed is used in a FEEDREADER one installs on there computer to keep track of new news threads etc.
Free RSS reader / RSS platform for enterprise grade solutions | Feedreader.com
The Photopost RSS feed is completely valid and much the same as the vbulletin feed used in vbulletin.
We code to the actual RSS VALIDATION and requirements. So validating our feed at W3C.ORG I show would be correct.
Your issue seems to be more of a how do I get it working with wordpress which might be best asked at wordpress support. I know nothing about what there specs are. In Photopost your dealing with a valid RSS 2.0 feed put out based on the RSS 2.0 specification. There is nothing really I show we need to change in our code.
Yahoo is its own RSS service so sure they may not care about validation and doing things like they should be done based on the actual specification.