Adding a link to an image
Adding a link to an image will depend on the editor you're using:
Drag and drop editor
Click on your image, and on the left side, you can insert your URL in the "Link" section:
Legacy editor
Legacy editor is not available on new accounts
Click on your image and select the Insert Link button. You can insert your URL there.
Alternatively, this can be done by editing the HTML of the email.