If it really an ISO and not a hard disk image, you need 'APPEND iso' (but then your file normally would have .iso at the end of the filename and not .img)
Code:
LABEL -
MENU LABEL Your own boot image here
TEXT HELP
Your help message goes here.
ENDTEXT
LINUX /boot/syslinux/memdisk
INITRD /ubcd/custom/myimagename.img
APPEND iso
How much RAM do you have? You need at least as much as the size of the ISO (a bit more if you want to boot this ISO).
_________________
Download
Ultimate Boot CD v5.0:
http://www.ultimatebootcd.com/download.htmlUse
Parted Magic for handling all partitioning task:
http://partedmagic.com/