You can add thumbnail images to your posts in two steps. First, make sure the thumbnail is enabled in the theme’s configuration file:

Then, provide an URL or path to the image file in the front-matter of your post:

The image path you put in the front-matter needs to be the relative path to the source directory of your website. For example, if you want to use the following image as a thumbnail:

You need to use the following as the image path:

Also, it is recommended that you put all the images under a dedicated asset folder that is separated from the _posts folder.