Hey @Louis ,
just wanted to follow-up as I’m correcting the site I’ve mentioned above and I thought, that you have implemented not only the option inside the link-selector but also inside the image dynamic SRC-Selector.
Just to avoid any misscommunication. Inside the image block the get_attachment_url() option is not available yet and will be available in a future update, right?
(Referring to this one here: Post URL Dynamic Link wrong value when querying attachment - #4 by Louis)
Thanks in Advance for your Answer! 
Cheers
Wolfgang