A Dermatologist is a Doctor specializing in the diagnosis and treatment of skin diseases. Dermatology is a branch of medicine dealing with the skin (which is the body's biggest and heaviest organ). It comes from the word "derma" which is the anatomic term for skin.
A Dermatologist may treat the skin picking disorder from it's physical aspects - healing scars, alleviating skin irritation, applying creams and give medicine subscriptions. A dermatologist will not (and is not qualified to) treat the psychic aspects of the skin picking disorder.