<media:thumbnail url="Resource/bmp/IMAGE_DISPLAY_USB.bmp" width="144" height="144" />
Change the media:thumbnail url to whatever graphic you want. The thumbnail currently used is in the /usr/local/bin/Resource/bmp/IMAGE_DISPLAY_USB.bmp. The width and height is determined by media:thumbnail properties.
Enjoy your jukebox and don't be shy to report any wishes / bugs I'll see what I can do.