It's all done with the magic of php. (See my Avatar). That's hypertext pre-processor. Here's a good example of one of these scripts if you are wanting to use one.
This one is pretty simple. If you wanted to spend the time you could change it to have a different image for BTS and ATS, a special image on Mondays.
Notice how my signature shows my current weather conditions. (if we're not in low-bandwidth mode) It will even show the current song I'm listening
to if I want it to. Your imagination is the limit.
WARNING WARNING WARNING
A word of caution. If you use one of these, make sure you size the images correctly or you end up with an avatar that's too big. If people were to
abuse this then I'm sure we'd lose the ability to use a script as an avatar.
[edit on 28-7-2005 by dbates]