click on "USER CP", then Press on "EDIT SIGNATURE" [it should be on the left side of your screen on the 2nd or 3rd row of the control panel- Its directly under 'edit avatar']
Then go to the Second option where it says browse [Click it] then go to the file where your picture is that you want is [Upload it] and then 'Save settings'
Also, the signature can Only be a certain size - if its to big it wont work
if your having a hard time send me the pic and i'll make sure its the right size and ill send you the Image Link so 'you could just copy and paste it'