Seems that uploading a custom header leaves the unused image on file, but does not provide the user a way to re-access it or use it. With a gazillion bloggers learning to use custom headers this defect in WPMU will eventually use a lot of space. A lot if there is no way to clean it up.
Seems that the folks at WPMUDEV would volunteer a mu-plugin that will automatically delete a prior image when a new image is installed. The script only appears to allow either one revert or a revert to defaults. The rest of this type are not needed and will consume gigs of HD over time.
Now there's my two cents worth. Ha! Take that and code it.