Click on your account name and then go to Modify Profile. Scroll down until you see the space where you can add a signature. In the space, put the link to the picture you want for your signature. But be sure to put [img] at the beginning and [/img*] at the end (without the asterisk).
All posts from my account that were made before July 29 2013 are from the point of view of my dad unless it states otherwise.