To change your Avatar, do what Hafize said above. To change your Who-Am-I picture, upload a new picture to the Who-Am-I gallery:
http://www.chiefdelphi.com/forums/pi...direction=DESC
The reputation system is very similar to the Karma system seen on other popular websites and forums (Slashdot.org, for example). Its purpose is to hopefully give users an idea of how respected, knowledgeable, helpful, gracious, etc., a person is. It is a system in which you rate your peers. It is a system that, hopefully, will make you think twice about your posts. You can read more about it in the FAQ:
http://www.chiefdelphi.com/forums/fa...n_faq#faq_rep1