Take a look at this senario: I wipe a HD with DBAN, re-format it, install an OS, I then use ghost to create an image of the wiped drive, place this image on a pc, will the pc with the new image be free of all previous data?
No, Ghost will only write where it needs to. It doesn't write over the free space.
You'll notice that your Ghost file is only as big as the "Used Space" after you installed the OS (or smaller if you used compression for the Ghost file). When you Ghost back to another disk, it doesn't bother writing over any "would be" free space. That's what's great about Ghost and others like it. It's fast. Even if it did overwrite the free space, it wouldn't be as thorough as using DBAN.