Here's an example of a collection of images, or an album. So I've started this as a means of collating a group of related items.
I wanted the extension to be as minimal as possible. As a result, I treat a collection post the same way as any other post: there's no additional information or details associated with a collection. I manage this by making albums that are simply 'responses' to other posts on the site with the in-reply-to field. This lets me give all the individual images additional information, by letting them be their own posts. It's a bit of a hack and I'm kind-of using in-reply-to fields for something other than what they were originally intended for.
It's just a mock up, really, but it does what I need it to do. I still need to figure out what the most sensible way to display all this is. :/