That's how I have my logo, in the sig.
Go to your profile
then to edit profile
then down at the signature part,
I used the

code to hot link it there from my website.
you should be able to do that with a picture from your gallery.
EDIT:
It keeps putting up the red x when I try to show you the code.
the code is [ followed by: img] then [ followed by: /img]