Adding a Link to an Image in Drupal 8
From Lisa Nugent
Add a hyperlink to an image
Select the image you’d like to add the link to.
Click on the Link icon in the toolbar.
If linking to a page or document in the site, begin typing the page title and permit auto look-up to find the page. Select the page from the found content list and Drupal will fill the URL field with the correct node or document path (node or file).
When linking outside the site, enter the URL in the URL field.
Scroll to the bottom of the page to Save.