It is a great idea to add your company logo into your signature as it communicates legitimacy to anyone who receives it and makes your postcards look professional.
To add your company logo, head over to Mail.
Edit any of your existing templates.
Click your Signature on the top right corner.
Edit an existing signature.
Scroll down until you see the option to upload a photo.
Be sure to save your signature.
Here's a video where Krista shows how to update your signature and add a photo.