i dont think you can but try this it works on a lot of places
say this is your sig
*picture you want in*
and it dont work... try this
[img]
*picture you want in*
[/img]
remember not to put a accual picture in ther but a link fo the picture, just right click the picture click properties and copy the url it is at. not sure if it works but you could give it a shot, dont hurt.