It appears that there is no way of updating an existing image from within the Draft Space; there is only the ability to replace it with a different image which is not the same thing.
Updating an image means creating either a new revision or a new version of the same GUID. Replacing the image means that I add a new resource with a new GUID to the repository, which, unless that is the effect I actually want, makes NO sense in a system designed to enable reuse.
When I replace an image that is a part of a reused data set instead of updating the image, I then have to go to every other place in the repository where the original image is in use and update that one, manually, as well. In essence, the workflow encouraged by Draft Spaces is forcing me to either branch of manually maintain my data set.
I know that the provision exists to be able to upload an update to a graphic in SDL because it's available from the Content Manager; please make this available from the Draft space as well.