Images cannot officially be added to OotW e-mail signatures.
Images cannot officially be added to Outlook on the Web (OotW) e-mail signatures. However, you can copy and paste an image that is hosted somewhere online. This effectively grabs the HTML of the image, which is then pulled up by the OotW when the e-mail is sent. It is important to note that the image must be hosted online - it cannot reside in your own computer