You can create a custom email signature to be inserted into 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 on the Track tab and scroll down. You'll see a section called, "Email Signature." Once you've customized your signature, to add an icon, click on the picture button in the middle of the toolbar.
Once you click on the picture icon, a box will appear that requires a picture URL.
To find an image's URL, the easiest thing to do is search on Google for an image of your logo or find a logo on your website. Right click on the image to copy the "link address."
Once you enter in the link address to the picture URL, you will have the ability to resize the image within the signature if you double click on the picture.
Make sure to save your settings after formatting your signature.