The point is, there is no universal solution that works for all e-mail clients. You can attach an image and that'll fail on some clients or be stripped off by some e-mail servers. And some people consider image attachments in the signature to be obnoxious. You can also create a fully HTML message with a link to an image, and that'll fail if the client has loading of remote images turned off (which is wise, since that has been used by spammers to verify opening of messages) or if the client can't handle HTML.
Ultimately, the best choice these days is still to just send a plain text e-mail, and if you need fancy stuff, put it on a web page and provide a link in your e-mail.