Mo,
You can use the delete commnad
switch# delete bootflash:
The only issue that you can face is that if sup reboots after deleting the iamge file, it will go into rommon.You have to take care that the Sup doesnot reboot or reseated in the chassis.
Please see the link below for upgrading the Sup.
http://www.cisco.com/en/US/partner/products/hw/switches/ps4324/prod_release_note09186a0080758ff3.html#wp466820
HTH,Please rate if it does.
-amit singh