You may recall that WordPress 4.8, released in June, added widgets to the core WordPress software for displaying images, audio, and video.
WordPress 4.9 will also include a Gallery widget, allowing for a thumbnail gallery of images to be displayed as a widget, which usually means in the sidebar or footer depending on your theme.
You can read the development announcement of the WordPress gallery widget which shows some example screenshots of the process of adding a gallery widget and choosing images (which uses the same media picker as you use when you add images to a WordPress post or page). As with everything else in WordPress, how it displays on your site will depend on how your theme styles the gallery widget output.
I Don’t Get Who This is For
While the gallery widget seems straightforward, I’m skeptical of who this is for… I doubt those of you who are serious about your photography are going to be manually creating a sidebar gallery featuring images from your WordPress media library. You’re probably going to be hosting your images somewhere more serious, whether that’s in a WordPress gallery such as Envira or on a separate photo hosting site such as Flickr or SmugMug. Both of those are excellent solutions and provide for more flexible output for your images.
This strikes me as a small bit of functionality that might be used by a hobbyist blogger1, but if you’re really into the best display for your photos, I doubt the simplistic gallery widget will be something worth your time. The gallery widget is a precursor to some block-style gallery functionality that will eventually be built into Gutenberg, the new WordPress content editor, and perhaps when that is ready to go the picture2 will become clearer as to how this will be used seriously.