Ok. What you need to do is find a way (with a zip disk, a floppy emu, a file server, whatever really) to transfer that image to a system 7 compatible machine. Once that's done, connect your external hard drive to that mac, start it up, and open the image file with Disk Copy 6.3 (4.2 can't handle hard drive images, that's why you need system 7...). Once the virtual disk's mounted on the desktop, just drag and drop its contents to your external drive.
Getting a file from the modern world to a vintage mac is kinda tricky. A floppy EMU makes things really easy. I cannot recommend it enough.
Alternatively, you could put your Floppy EMU in HD20 mode and boot from that .dsk directly.