To do that click the "post an update" button and then the camera button to include the photo in the post.
Hmm. I should make that a bit more streamlined. I found myself wanting to do the same thing on my ride yesterday and it's wasn't as smooth as I would like.
When you upload the photo there's the title and description fields which are awkward.
When post a photo you can add comments to it afterwards.
Or you can embed the photo into a post which looks better on the timeline will only be visible there.