If you have photoshop, you open the picture you want to use, crop it to your liking then open a new file, set the pixals to 72 x 72, go back to your picture, ctrl a (select all ) ctrl c (copy) and go back to the new file and do a ctrl v (paste) ctrl t ( to bring the sizing tool up) hold down shift and drag the left bottom corner up until your picture is perfectly in the box. Click the move tool button and it will ask you to apply. Click yes.. then file --> save for web and make sure its under 7k (if not you can lower the optimization level until it is) then save.
If you dont have photoshop well.. *shrugs* lol.
Someone can help you.