This is probably a bit late now but I've been having the same trouble and have been smashing my head off the desk for a week, but I have come across the answer, finally!
It's not a problem with the .img file it's the version of memdisk you are using, download the latest version of syslinux from http://syslinux.zytor.com/wiki/index.php/Download navigate to the memdisk directory and copy the binary from there and replace your old copy, job done!
Hope this helps someone!