Email Templates: How to Add a Hyperlink and an Image to Text

• First, type your text in the email template body. Highlight it, and select the "Link" icon from the Formatting Controls bar:

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• 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. 

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Salesforce Source: Learn how to add a hyperlink to an image or text in email templates