[use the search method please next time, there are alot of people asking this]
Anyway, a sig is a pic, usually a size of 400X100, but in AO case, the maximum size is 500X115, plus a 500X20(userbar), and a clan bar. ALso is only limited to 4lines of additonal text(spaces count as lines).
to put an image onto a board you have to use bbc code which is like : [img] , that, then finish the url(of the image of choice) with an "/" before the "i" this time.
to upload a pic/sig, you need an online image hoster such as
(just easy as sign up, upload and copy the url(adress).
no matter how many times people ask, i always answer, and my answeres get longer ech time :P, i'll keep doing it though
this also may remotly help:http://img296.imageshack.us/my.php?image=sigtut5jm.jpg