Extension name: Lightbox
Author: MattF
Extension description: Adds the Lightbox2 jQuery plugin to phpBB. It will resize posted images down to your defined maximum height/width settings and display them full-screen in an elegant Lightbox overlay effect when clicked on by the user.
Features:
Resize images posted using the img tag or uploaded as attachments (resizing occurs in the browser)
Resize images in user signatures option
Images can be opened in a full-screen Lightbox overlay
Gallery mode option in the Lightbox overlay
Adjust settings in ACP->Post settings->Lightbox image resizing
Lightbox effect and resizing disabled for mobile devices where its not needed
Extension version: 2.0.4
Tested on phpBB version: 3.2.11
https://www.phpbb.com/customise/db/extension/lightbox/