I'm still really interested in this hack, and it looks like there's other interest from at least kuwaitsun as well. Anyone want to take this one on?
Mist has created
this nice hack which works with the PHP vb code tag in messages, but he's a bit busy to take this IMG hack on.
For admins who host a lot of images in posts, this seems like an excellent way to reduce bandwidth AND get new registered users. Ideally, I'd like to see it be able to have an ON/OFF toggle per certain forums, so it would be easy for Admins to decide which forums get the treatment of this hack, without having to alter more templates, etc.
I'll be happy to create the graphic(s) needed to show the "Sorry, you must register before you can view this image." graphic, etc.