First, select the area you want to use, and "crop" it, so you don't have lots of image that you don't want, if you haven't already done this. Then, "Resize" it. Look in the edit menu, or image menu, depending on your program. For a big avatar, like Marnie's dog image, you can do 150 pixels wide. For a small one like mine, you can do 60 pixels wide.
Good luck! If you have more questions, let us know what program you're using.