I've got the first slice of this feature coded up, just needs a bit of tidying.

I figured that it'd makes sense to break the work down into two parts:

1. Done: synchronously regex check and format urls that have a recognisable image url format
2. TBD: asynchronously check urls and format them as images if the mimetype returned is an image

i'll open up a pull request to get some initial feedback and if you'd prefer not to merge until both parts are completes, that's your prerogative

jowo

There will be more bounties and if someone beats you to it yet your work is solid, I'll give you a partial bounty for the effort.