You know those cute little pictures which pop up if you receive an email from somebody with a Mac.com account? It turns out that you can set that picture with an email header: no Mac.com account needed.

Simply add a X-Image-Url header to your email, with the contents being a URL that contains the picture, e.g. X-Image-Url: http://www.algorithm.com.au/albums/ozone/Me.jpg.

Credit where credit is due: I found out about this tip from the following two links: