Hi Deb, there is a way to do this when you are starting a new thread -- kind of.
Start in the collection itself. That is, go to the page of the collection that you want to draw from. Then click the "Post About This" button. That will open a new "Start Conversation" window with the finds from that collection pre-populated in the attachments. You could then remove any of the ones that you don't want in that post.
You can also add items in the normal way from your full collection.
This isn't ideal, because it starts the post with ALL items from the collection so you have the chore of removing ones you don't want. And you can't easily draw items from two or more collections. And, you can't use this to add items to a reply.
Does this make sense?
I will think about how we can add an easy way to select from collections when we make improvements to the way you pull items into a post.