Can I widen my narrow smile with veneers on the back teeth (premolar and molar) only? (photo)

The photos show the narrowness of your arch from the front view quite well. To answer your question, there are several options. Further orthodontic arch development can still be achieved with an LVI dentist/orthodontist trained in physiologic orthodontics or at Dr. Gallela’s Facial Beauty Institute. There are techniques and appliances that these practitioners can apply to get that robust arch form you are looking for. It would, however, take about 2 years in appliances/braces.
From a porcelain standpoint, though it would be more ideal and complete to have 10 units of porcelain perfectly matching with optimal widths, it could be done in less. I think you’d fall short with 4. Including the canines also (making 6) would go a long way as this is THE transition tooth between the front teeth to the back and it sets the tone for the angle and thickness that you desire on the back teeth to fill the space known as the buccal corridor. 
As for no prep veneers, maybe. Most of your goal is additive in nature which makes this a possibility. Sometimes very minimal prep can be more effective and dependable in this type of instance if you are open to that. That decision would involve looking at stone models and more photos to make the call.