This looks like a very useful piece of work.
One thought that occurred to me -- should we make any provision for information about previous image revisions?
Some applications (eg georefencing) may have produced data related to a particular revision of the image. Is it worth storing any information about the old image (eg dimensions), or about any best-effort transformation to map its content to the current image or the reverse?
I am not sure it makes sense to mix historical data with current data. I think for now it makes sense to focus on representing the current state, and deal with history later, if the need arrives.
Thank you for your review! Indeed it would useful to allow to query data from previous revisions but it is not something specific to image metadata. It would be nice to be able to query all data from previous revisions. I have made a first attempt at it for Wikidata item contents. This system does not care about how actual content is represented (items, properties, mediainfo...) as soon as it is RDF triples from a MediaWiki revision. So, generalizing it, or an other approach, to Commons should not be hard and cover the use case you are mentioning.