A basic gallery extension that's "slightly" better than the Mediawiki default gallery.
Once you've downloaded the file, upload to the extensions folder and unpack, and finally add the following to LocalSettings.php
require_once( "$IP/extensions/Gallery/Gallery.php" ); $wgAllowExternalImages = true;
You'll also need an external image host, Cloudinary are one of the best, they're free and you get 10GB of storage (1000+ images) and 20 GB per month of bandwidth.
<div data-darkbox="https://res.cloudinary.com/mediawiki/image/upload/v1516740587/beach-1.jpg" data-darkbox-description="Note that you can also add a description"><span class="img-thumbnail">https://res.cloudinary.com/mediawiki/image/upload/c_scale,w_250/v1516740587/beach-1.jpg</span></div>
Note above that Cloudinary allow image scaling so our main image (the popup image) is as is, whereas the thumbnail has been made smaller with