Lest anyone thinks that this query went unanswered, many thanks to @zogstrip for solving this. It seems that Discourse got confused about whether those uploaded images were being used and moved them for subsequent deletion.
Though you will probably never need it, the fix went something like this:
# copy the "deleted" images from their to-be-deleted staging area
cd /var/discourse
./launcher enter app
rake uploads:recover_from_tombstone
# rebake to see that it doesn't happen again
rake posts:rebake