After thinking about this, here's my reasoning why phpBB should not add a lightbox overlay effect to posted images to the core (and this isn't because I just made an extension to do it, that was just done as a proof of concept as to how easy one can be integrated now with the changes made by this ticket)
1. Lightbox overlays are almost always used to display a full screen version of an image that is either a thumbnail or text link. Images in posts are not thumbnailed (except maybe for attachments). But for your average large [ img ] posted image, which will scale to fit the full post area, there is nothing to be gained by opening it an overlay. phpBB is already full screen, so the image isn't going to be revealed in the overlay as anything much bigger than what it already is in the post area. In addition, the image will still be scaled to fit the browser window, so it is not necessarily going to be shown at "full size" either.
2. The overlay itself. There are dozens of lightbox plugins out there. It's a wheel that does not need reinventing. Licensing issues could totally limit (or prevent) us from using a really great overlay plugin, or force us to invent our own (which seems way over the top considering point #1). By leaving this type of functionality up to extension authors, frees them up to offer versions that use a variety of available plugins, giving users more choices in what they may want to go with for their board.
3. Extensions. Since extensions could possibly offer better overlay plugins and options, it would be best to give them the room to do so. I think if the core had its own JS overlay stuff going on, that could collide with the attempts of an extension to integrate another overlay plugin.
4. Mobile/Repsonsive issues: The lightbox overlay is not even needed on a tablet/phone device. The current handling of images now, presents them perfectly on these devices. An overlay effect would seem more in the way on these devices, for example, trying to save an image may get messy if it is instead trying to open in an overlay.