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 should be able to right-click on an image (or click a drop-down associated with image, or whatever) and be given the opportunity to actually update the existing source.
Currently, unless I want my web users to spray a firehose of redundant images into the repository I need to direct them to take the GUID of the image from the attributes panel of the Draft Space, open the Content Manager, use Locate to find the resource, and use the Update feature of content manager to update the graphic. That's the kind of thing that makes people question whether or not I think their time is worth anything.