We proceed to put our username and password to login gmail.
Then go to Settings, Gmail's new Mail interface will click on the icon of gears.
We click on Settings
We will see several options in this menu, near the end we will see the signature option where we have two options, select signature
Select this box and see how to activate a rich text box where we can customize our firm, post images, etc.
Once we have our firm we click on the "Save Changes" and ready, whenever we send an e-mail the custom signature.
More information at www.gmail.com