Airmail — 6 steps, ~2 minutes

Copy button included below

1
Open Airmail and choose Airmail → Preferences from the menu bar (Settings, on newer macOS versions).
2
Click the Accounts tab and double-click the account this signature is for.
3
Click the Signature tab, then click + to add a new signature.
4
Click "HTML" in the signature editor to switch it to HTML editing.
5
Paste your signature's HTML source code into the editing column — Airmail shows a live preview alongside it.
Make your signature first →
6
Click Save.

Good to know

Pasting the HTML source, rather than the signature itself, keeps your images as linked images — a plain rich-text paste into Airmail can send them out as attachments instead.
Airmail keeps signatures per account, so repeat these steps for every address you send from.
Airmail for iPhone and iPad keeps its own signature list: tap the menu icon in the top left, then Settings, tap the account, then Signatures — and paste your signature there too.