You can add your signatures on Alto for them to appear on your emails and letters.
Go to Tools>System Administration> Group Users.
Click Edit on the user and scroll to the bottom of the page. Click Upload New File.
Add in text for the Caption, for the Media Type select Public Image
The Sub Type should be changed to either Signature Images for Letters or Email Signature.
Next click on Choose File and navigate to where the file is stored. (The signature should be in a JPG format)