Oh. Let me teach you! :3
Go to settings, then go to "Edit Signature".
There is that little box thing there, right? And then, click the little tree photo there.
You will be asked to add a picture there, right? A URL picture. Your signature must be smaller than 800x250 pixels..(widthxheight, i think)
Well, feel free to do anything with your signature and then press "Save Signature".
Hope that helped! If you didn't understand, VM me about it.