In addition to adding email signatures to CareerPlug's email templates (more info: How do I create a default email signature?), you can also edit your message templates using HTML. Head to your Account tab, click Manage Hiring Process and click the 'Message Templates' link on the left-hand side of your screen. Then select the template to edit:

User-added image

Use the formatting toolbar to bold, italicize, add bullets, indent paragraphs, or insert photos and hyperlinks. 

Editing HTML in message template