How do I edit pictures in Adobe Illustrator CS2?

I need to edit a picture which I downloaded from the internet. It is a picture of a woman running and what I want to do is to keep the shape of her but to get rid of the inside shadows and background. The problem is that whenever I press the brush on, is comes out with a little symbol showing me that it is impossible to do such thing. How am i suppose to draw anything? Can someone please help me, and tell me what am I doing wrong?