I have found this to be the easiest way to delete image packages from all site servers.
Login to your NS server, go to the image path directory on your NS. Go to the directory of the image you want to remove, delete the file. Open notepad and then save a blank file as a .GHO file matching the same name in that image directory.
Go into your console and view the image package source to make sure it shows 0 KB, then update distribution points.
Presto, disk space returned in a nearly automated fashion.