Thumbnails can be enabled easily by setting the thumbnail parameter in the frontmatter to an image such as "images/landscape.jpg".

Make sure to copy the image the static/images/ directory.

If put together, it will look like this (that’s in fact this post’s frontmatter):

+++
author = "Hugo Authors"
title = "Guide to Thumbnails in Hugo"
date = "2019-03-04"
description = "Guide to Thumbnails in Hugo"
tags = [
    "thumbnail",
]
thumbnail= "images/landscape.jpg"
+++