The signatures for your sending accounts can be configured from the account settings. Edit the signature in accounts settings, and then import it into your email with the variable {{accountSignature}}.
Here are the steps to add signatures:
Go to Accounts dashboard, and open the account settings of the email account. Here you will find the signature editor.
You can edit the signature as per your liking.
To insert a signature into the email copy, go to your campaign sequence editor, and click on the Variable button.
Select "Signature variable".
Note: On step number 2, if you'd like to add an image to your signature, we suggest you read this article on how to do so with Google Photos and Labnol.
And there you have it! This variable will import the signature for every sending account you have connected to that campaign.