The "now playing" part is a pure web interface and thus its appearance 
is under control of Remote Buddy/AJAX Remote.

The Quicktime / Web Media Player on the iPhone, on the other hand, is 
built-in into the iPhone and can't be controlled by either Remote 
Buddy nor AJAX Remote.

Currently, that Quicktime / Web Media Player will not show any cover 
art for any music file. It's simply not supported in the current 
firmware. It will always show a QT logo with the URL of the source for 
all audio and only display something else for video playback (would be 
bad if it wouldn't ;-).

Best regards, 
Felix Schwarz