You need photoshop or equivalent, go to Image -choose image size once you have it up and change the height to 50. It will fill in the blank for width (to 25 or 30?) which will make your image pretty small. If it was square it could be 50 x 50 but your image isn't. Save it where you can find it. Then log on and go to user CP at top, choose profile options, and scroll down to avatar, choose browse and find it where you saved it, hit enter and voila!
Little Alex tried to explain it to me months ago, and I've just now figured it out. Thanks Little Alex, wherever you are! Finding a pic that is recognizeable when its that small is the hard part, your image is great so I suggest you do try it! Maybe crop some of the hunter?