In reply to Aaron Davis.

Happy New year to you too Aaron,
It is WordPress a plug-in that lets you use a shortcode, with a lot of parameters. You can add some CSS to style things (my photo page uses it too). The books pages uses it.

I think it might be my favourite plug-in. The plug-in site has various examples and ways of extending it. Eg on the photos page and
https://johnjohnston.info/blog/myphotos/butterflies/
I’ve added a snippet from the site to my functions to show the first image when there is not a featured one.