Easy! Just click on the link to "User CP" up at the top. You will see "Edit Profile Picture" which will be the image people see when they visit your profile. you also have "Edit Avatar Picture" which is the picture people see on your posts around the forum. When you click on those links, you will see the menu for uploading.
BTW, this should be in the User Guidelines & Forum Features section, rather than Introductions.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein