You mean an avatar/icon for your profile?
All you need to do is have a small pic ready, you can crop or resize images in any programs like GIMP, Photoshop, Paint, etc.
I think 100x100 is a preferred size for the forum but I'm not certain.
Then go to Profile,
Forum Profile Information,
and select "I will upload my own picture:" and choose your small image file, and then once you're done, click Change profile at the bottom, and it will upload your photo onto your profile.