Thanks for your questions! The Portal Signature found at Settings > My Settings > My Preferences > Options tab > Portal Signature is used on the Portal only. An example of this is my signature at the very bottom of this message. This will only be seen in Community threads and replies.
To set the agent signature(s) that is used for ticket responses, you'll go to Settings > Configuration > Tickets > Signatures tab. You can create one signature for your entire brand or separate ones for each department. You can also utilize variables so agents can have custom signatures.
Here's how we utilize them here. We have one signature that is applied to all departments and groups within our brand, and along with our other general information, we use variables to customize this signature per agent. The variables we use are:
[#agentavatar#] - to show each agent's picture
[#userdisplayname#] - to show the agent's display name
[#departmentname#] - to show the department the agent is replying from
[#companyname#] - to include the name of the Brand (SmarterTools)
So if I send out a ticket response from the Sales Department, my signature will look as shown below. This is how all of our signatures look - just customized to the agent and department selected.
I hope this helps!