How to create a signature with embedded image

First of all, I know how to create a signature. The thing is, I want to include an image in base64 or as an attachment. Why? Because most mail clients don't load external images by default, to protect the user's privacy.

I tried adding an img tag with the base64 encoded image but upon save Carbonio sanitizes the input and I end up with an empty img tag.

Anyone knows how to get around this?