How can you treat a narrow smile? (Photo)

Hi:You have two choices:1.  Move your side teeth more outward with orthodontic treatment2.  Place veneers on your side teeth to make the arch wider.Best,Dr. Maddahi

Narrow jaw can be corrected by Orthognathic surgery, Lefort 1 osteotomy. Orthodontics are needed as well. 

Based on your photo, you appear to be a good candidate for either braces for better alignment or porcelain veneers. Veneers can improve the shape and color of your teeth, and ultimately make your smile wider and fuller (by adding porcelain to your teeth). Seek an experienced cosmetic dentist who has great photos and reviews.