Been doing some testing on this.
For outlook mail client, the image is sent as an attachment then it automatically embeds it via html source code
For gmail, the image is loaded remotely on the googledrive via html source code
The problem with smartermail currently is signature only allows for either
a) load from the profile pic using variable
b) insert a remote picture link remotely
I'm suggesting an improvement on the signature editor where you could link an image from the user's smartermail file storage (similar to Link From when creating a New mail).
Currently, the signature editor is completely not user-friendly for a non-techy person, it is hard (almost impossible) for them to find the "URL" of an image then paste the code it. Whereas if you have a outlook client you could just drag and drop the signature.