How can I pick a default signature to be used for the Agent?

To select a default signature to be used for an Agent, first you'd have to create an HTML-based signature for each agent by going to Manage --> Agents, click Edit and in the "Hooks for Email" area, add an HTML-based signature in one of the 4 signature fields.

Once you have a signature in at least one of the fields, you can choose from the dropdown "Default Signature" which email signature you want that user to have as their default.  Scroll to the bottom and click Save.