At Exclaimer, we see a lot of email signatures. We see what a lot of companies get right and what they get wrong.
DO restrict image dimensions
Never assume that the size of the image is correct. Ensure that the height and width are set within the HTML so Outlook does not render the image strangely.
Again, for all the non-coders, don’t just use a web CMS or Word to try and draw out your signature .