I have been doing a lot of fiddling with hard drive image files and gparted and I'd like to share what I've figured out.

You can make an image file of a hard disk with dd like so:

dd if=/dev/hda of=/mnt/a/MYIMAGE.IMG

then you can mount it using a loop device with losetup:

losetup /dev/loop0 /mnt/a/MYIMAGE.IMG

then you can run gparted using that loop device, specifying the devices you would like to use:
so if I'd like to copy partitions between an image file on /dev/loop0 and my hard drive on /dev/sda:

gparted /dev/loop0 /dev/sda

also if you have multiple loop devices you can use:

gparted /dev/loop0 /dev/loop1 /dev/sda

and when you wish to disconnect from the loop device just use:

losetup -d /dev/loop0

One thing I should mention: Knoppix 6.3 seems to require specifying loop.max_part=8 as a kernel parameter on the command line to get gparted to work right. Knoppix 6.2 doesn't seem to require this to work properly. Perhaps the version of gparted with 6.3 is a bit more finicky.

Anyway, have fun imaging and editing with gparted.