The reason there are __MACOSX folders when unzipped, is because the two pictures have resource forks. This is a legacy of the OS 9 days but still supported under OS X ... which haunts us Mac users from time to time
Most unzip utils like JIMZip for Mac (WinZip for Windows?, what did you use to unzip Chuck?) don't recognize this and don't attach the data fork and resource fork together so create a __MACOSX folder with the resource fork files in it, and that's where PhotoPost stumbles as Chuck mentioned.
To solve this, zip your files either with DropStuff and uncheck the 'Preserve Macintosh content' setting in it's preferences first.
Or use a utility to remove resource forks from files you wish to create a archive from, take a look at some over at http://www.macupdate.com/
to find such tools
After all resource forks are not needed and just contain a preview of the file, some version strings and stuff like that.
Hope this helps ... it helped me after some checking, cause i was bitten by the same error at first