Well. slw, it's around 10:45pm for me, but here's the general idea of how you do it.
Firstly, you have to get the picture from the phone onto your computer. One way to do it is to email the picture to yourself from the phone, and then collect the mail with the picture on your computer. If your phone has, say, a capacity for you to transfer the picture directly onto your computer via a USB cable, that's another possibility.
Secondly, you need to put your picture out on the 'Net somewhere from where it's publicly viewable. The simplest way is to join one of the free image hosting services (such as Photobucket or Imageshack, among others). You upload your picture to the site according to their instructions, and they will in turn provide a URL (http something-or-other) by which your picture may be found.
To make that picture your avatar, you go to your profile editing area on this board. Under Personalized Picture, you select I have my own pic, and fill in the box on the right with the URL to your picture as provided by your image hoster (Photobucket, Imageshack, whoever). Then you click on the Change profile button.
Your avatar picture will be displayed on this board at a size of 80x80 pixels. Unless your original picture was square, it will appear distorted (stretched in one dimension and compressed in the other). To avoid such distortion, you need to use an image editor to crop your picture to a square shape before you upload it to your image hoster.