If you wish to add a signature and/or logo or image to the bottom of your emails, where and how you set this depends on how you send emails.
If you use Webmail
you can add a signature in Settings > Identities > [click your address] > Signature. You can turn on 'HTML signature' if you want font/formatting options or to add an image/logo. Once you have saved your signature, the signature button on the compose page will become active.
If you use email software on your computer/mobile device you should set the signature within your email software as per your software's instructions.