How people see you VS How you see yourself
Body image can be very effective on many people especially teens. Its not you, its your brain telling you that you have too much of this or too fat of this. For example: many pimples and you would call yourself ugly. Another case of Body Image would be you comparing yourself to other people such as your favorite actor/singer/person you look up to/friends/the "popular" girls in school.
When people describe how you are they are less descriptive than if you were to describe your self, they do this because they focus on the good things that are on your face/body less than the little nooks and crannies. I also think its a great idea to have someone describe you and then describe yourself to someone so they can draw you.