This is a great extension and a must for companies like the one I work for, that demand users to send email with HTML signatures.

The problem is, currently, only external images are allowed and many email clients block such requests. So, the extension could upload the signature image in the background, a few seconds after you open a compose window, so you could put a <img> tag inside your html code referencing the mime-part of the uploaded image and display it inline. :) Something like:

<img src="$SIG_ATTACHMENT_ID">

Where $SID_ATTACHMENT_ID is the X-Attachment-Id of the message (generated by google every time you upload a file).

Cheers,