Adding signature custom field
To create a perfect footer, you need to go to Settings → Integrations, choose your email account and click on Actions → Edit settings. There you can fill out your signature - if you use Gmail, you can even import your signature from there! Moreover, on the left hand side you can fill out all your sender custom fields as well.
Once that's done, you can add your signature (and other sender custom fields) in your message templates.
Just go to Messages → Templates, choose the template to edit and add the sender_signature custom field to your message (if you're not familiar with the concept of custom fields, check this article).
Adding logo to your signature
You can also add your logo to the signature - simply upload a picture to a hosting site such as Postimages.org, and in the signature creator click the image button, where you can paste the link to the image and define its size. Here you can read more about uploading pictures.
Voila! Your signature is ready to be sent!