First your picture needs to be somewhere on the web. You cannot link files that are on your computer. If you don't have anywhere to put them on the web, you can put them in the Cat Photos section of this site.
Do you mean the small picture just under our names? That's called an "avatar." If you would like one, you will need a picture that is no larger than 120 x 120 pixels and no more than 6 KB.
When you have that on your computer, click on the profile link at the top of this page. In your profile, scroll way down to the bottom until you get to the "Avatar Control Panel." You can upload the picture straight from your computer. That's how I got my avatar.
I took the liberty to optimize the picture for you. You can save it to your machine and upload as your avatar. Let me know when you've done that and I'll delete it from my gallery.