You can make your own, it really can be a jpg a gif a png or a bmp.
But if you want a choice of selections to download, there are a couple of AIM icons web sites. It just so happen that XP's user icon is the same size as AIM user icons. If you go to this site http://www.gigahosts.com/~c1003/aimicons/ I think you'll probably find just about anything you could wish for.