Outlook on the web or new Outlook: Settings (gear) → Account → Signatures → New signature → name it → paste → Save. Set it as the default for new messages and replies. Outlook on iPhone: the app only takes plain text signatures. Set it on the web first; mail from your phone can use a short plain line. Gmail: Settings → See all settings → General → Signature → Create new → paste → Save changes at the bottom.
Paste with a normal paste (Cmd+V or Ctrl+V), not "paste as plain text", so the logo comes with it.
