Do you mean add an avatar? Go the the blue band above the thread list. On the far left, click on "User CP." A drop down menu will appear and then click on "Edit Avatar." It will take you to the page where you can add an avatar. There are several to choose from here, or you can add your own.
If you mean adding a picture to your personal profile page, again, go to User CP and click on "Edit Options." There you will find a link to add a profile picture.