- First, type your text in the email template body. Highlight it, and select the "Link" icon from the Formatting Controls bar:
- Enter the URL in the popup that appears. Click 'OK', and the text will turn into a hyperlink.
- Save the template.
- Click 'Edit' again, and highlight your hyperlink text.
- Select the image icon from the formatting controls bar and insert your desired image.
Salesforce Source: Learn how to add a hyperlink to an image or text in email templates