Select the signature as it appears on screen (the rendered version), then copy with Ctrl + A, then Ctrl + C.
Open Outlook (desktop app).
Go to File - Options - Mail - Signatures…
Click New, name it, then click OK.
Click into the signature box and paste with Ctrl + V.
Set defaults for new messages / replies if you want, then click OK.
Important: Outlook won’t reliably accept raw HTML pasted directly. Do not paste from the “Output HTML” box - paste from the browser-rendered signature instead. Images may appear after your first send.