Click on your name on one of your posts or at the top of the page, then above the blue box it shows some options, choose edit my signature and then type what you want in the box. Also when you get a pic for your sig click on IMG then put the pics URL in the box then hit ok.
it looks like you already have one, but if you wanna change it just go to avatar options in the same section as your sig and put the URL or upload a pic.