Hi kgm,
You can sign in Outlook Web App (OWA), send an email to yourself, and insert the email in this email. Once you receive the email, copy the picture.
Besides, you can also type the URL in browser and open the picture directly. After this, please right click the picture and click copy.
Then click Options -> See All Options -> Settings -> Mail, use Ctrl + V to paste the picture in E-Mail Signature.
Thanks,
Young Yang