You can create a custom email signature to be inserted at the bottom of your outgoing Track emails. First, click on your picture in the upper-right-hand corner and go to Settings. Once you're in the Settings page, click the Track tab and scroll down. You'll see a section titled Email Signature. Once you've customized your signature, click Save Settings at the bottom of the page.
You can add pictures to your email signature too, simply follow the instructions here.