Link to website from thumbnail

If you’re on mydomain.com/listing/my-listing, I have a thumbnail of the website that’s listed.
You can click on the tab ‘Details’ and then click ‘Visit Link Here’ to visit the website.
What i’d like to have is clicking on the thumbnail takes the visitor to that website.

The documentation isn’t very clear and sometimes even contradictive on how to do things in a child theme. The child theme is in place, but i don’t understand how to do this.
If it read the docs, i feel like i have to change _single.php, but that file is not available in the child theme, nor in the parent theme.
Any ideas?