I was getting the message "not a COM32R image" when trying to boot from a USB flash drive created with usb-creator-kde.
I found you can work around this by typing "help" at the boot menu, and then pressing enter. Ubuntu will then boot and you can continue from there.
http://www.linuxquestions.org/questions/linux-mint-84/trying-to-boot-linux-mint-9-from-usb-flash-drive-vesamenu-c32-not-a-com32r-image-829397/
