It seems that there is no way to do that. It is also very annoying to try making a signature file in html, more if you want to add an image (a logo) and you don't want to have it with shadows because shadows are set by default. The only way that I have found to do that is adding the images in the background with the line:
<div style="width: 186px; text-align: left; height: 16px; background: url('C:\Users\Boss\Documents\Signatures\mylogo.jpg') no-repeat"></div> Sorry that I wasn't o help to you but I guess that you have been fighting with the signature html files to avoid the shadows on the images. Hope it helps.