Why is it important for teenagers to have both a positive self-esteem and body image?

It isn't. The so-called "Self-Esteem Movement" has done nothing but wreak disaster upon an entire generation raised to believe that they are better than anyone else at most everything. The result has been self-obsessed individuals whom no employer wants near their company and and an epidemic of suicides by people who eventually discover that they are none of the things they were told to believe they were. Objectivity went out the window and mirrors become the sole judges of reality.

The fault, dear Horatio, lay in the fact that "self-esteem" is a ridiculous concept, logically circular and self-referential. What should have been emphasized was each individual's duty to their family, their group, their town, the people they love, and their country. The image people have of themselves is ultimately the image each person believes that they have for others. "Self-esteem" missed the part about the fact that looking at oneself, one is really looking at oneself as a third party, i.e. someone else.

Body images, more than being positive, need to be objective. The truth, though it may hurt, is the categorical imperative and it will keep coming back, keep popping up, and refuse to die, until people face the fact that the truth is really all that there is.

Anyone now using the term "self-esteem" is at least 2 decades out of the loop and hasn't been paying attention.

Well, I'm a teenager. It is very important to have those things. I know this, because I didn't before. I hated myself. It's really hard when you hate yourself. I considered suicide multiple times. And I almost was anorexic. But, now I have a positive self esteem and body image. I'm more confident and happier. So, that's why those are important. I hope I answered your question