IMO, ideal would be for the quoted message in the reply toThat would be feasible in the web view of the messages. The reply would have the URL of the original image placed in its HTML <img> element, replacing whatever src reference it had on arrival.
When the reply is sent out to members by email it may be best to leave the quoted image as it was received, to avoid the various ills associated with sending it as an https access.
I have no clue how coding would accomplish that, but we're using codeDifferent types of difficulties, but yeah.
Were Groups.io to store a hash value with each image it ought to be pretty straightforward to see if any image file incoming in a reply matches an image file already stored in this group (or topic).