The default interface for Mail signatures allows you to do rich text signatures using the fonts and colors palette. You can even drag an image into the compose signature window and it will be included in every email as an attachment. This is fine for most people, but attachments should be just that, an attachment of a file I am sending, not an image in my signature. So here’s an easy guide on how to do CSS signatures referencing images on an outside server and not as an attachment. This tutorial covers Apple Mail in Mac OS X Tiger v.10.4
[Source: digg.com]