To get started, visit your profile page at http://music.aol.com/profile
Make sure you are logged in.
Right under your profile picture will be a link to "Edit Icon". After clicking that, a window will pop up which will allow you to select an image from your computer, crop it and then set it as your new profile picture. In addition to your profile picture, this will replace your AIM icon as well.

Please note that Flash 10 is required for this.