People who have such signatures will tell you where to get them, but the mechanism is simple - you hot link to an image. That image is then downloaded by the guy who is viewing the web page. When the browser requests the image, the host determines which browser, IP etc. (there are simple ways to do this), generates a dynamic image and sends it back.
Another mechanism would be javascript, but I don't think anyone uses that (I doubt any forum would allow javascript).
There is no way to get a MAC address - the MAC address is not transmitted to the remote host.
