Yes, you can! When you are creating a Campaign, you will see at the bottom of the touch the option to insert a signature. If you haven’t already, you can create your signature by selecting the "Create" button. From here, you can re-create or create your signature and insert images and links to it.
Once you have created your signature, you can choose to have your signature included for each touch by toggling the "Include Campaigns Signature" button to the right.