It appears Yahoo Messenger for Mac is no longer supported by Yahoo. In the past 6 months Yahoo has changed it's protocols such that Mac versions of the Messenger app can no longer receive files from later PC versions developed after that date (they can send fine). I have pursued this issue with Yahoo's tech support dept, who have shown no interest in bringing the Mac version up to date, or in further supporting the Mac platform. Unfortunately until the new file transfer protocols are reverse-engineered, all third party IM apps accessing Yahoo using GAIM (hacked access to Yahoo's IM), suffer the same file transfer issues with newer PC versions of Yahoo IM.