If you are using the related content feature on your site, you will see a post list at the bottom of each page which links to other posts using similar tags to the current post.
If you are using a countries taxonomy list, the first thumbnail in this post list will be a link to a search page, filtered by the country tag used on the current post.
For example:
To add a thumbnail image to the country tag:
- Open the countries/locations taxonomy list on your site: [your domain]/wp-admin/edit-tags.php?taxonomy=location
- Click on the country to edit the term
- In the basic settings section, add a featured image by clicking choose or upload an image
- Add an image from the media library, or upload a new image, and click use this file
- Scroll to the bottom of the page and click update
