Hi @samgrover! Looks I missed a couple things in the documentation. Yes, to get all the photos in a collection, pass a new microblog-collection to the /micropub/media?q=source endpoint. The collection is the URL returned from the list of collections:
GET /micropub/media?q=source&mp-destination=...µblog-collection=https://example.org/collections/1
You can’t add a photo twice to a collection. The photo URL has to exactly match the primary photo URL returned from /micropub/media?q=source. It can’t be a CDN or an external URL. It has to be the photo URL as Micro.blog knows about it.