Click on [change avatar] at the bottom of your [edit options] screen. Either type in a url pointing to the picture you want to use, or click on [browse] to find a picture on your own computer to upload.
Yes, I believe touristguy did come forward. www.touristguy.com published an email along with some pictures taken at the same time on top of the WTC. The one I use for the avatar is from one of these, and is noticeably different from the famous one. The face is turned slightly to the left of the camera and shoulders are turned way to the left. Apparently a news outlet reported that he was Brazilian, but the email said he was Hungarian.
Here's the rub, though. touristguy.com disappeared from the web only a few minutes after I downloaded the picture yesterday. My browser does not return an error, just a blank page. If anyone gets a different result let me know. The domain is registered to a bogus name and address.