Good morning Jonathan,
it seems to me that having reusable images in a dedicated warehouse-topic is easier to manage than keydefs in a map: create a table with two columns where the left column holds your images and the right column displays the assigned image-ID and holds additional comments for context, if needed. Reference with conkeyref.
Keyrefs, on the other hand, seem to be well suited for what I like to call "content swapping": multiple keyscopes contain the same key with different hrefs, content is "swapped" depending on the keyscope. This is useful for cases where entire topics or parts of a topic are reused but the image needs to be different in each instance (this method, of course, is not restricted to images). Beware though: if you decide to use keyscopes, you’ll not be able to properly use reltables alongside with the current DITA-OT (3.5.2).