When individuality is being emphasized (that is, ‘not one’), a singular form of the verb is used:
“She is difficult to describe physically - none of her features is particularly striking.”
(That is, of all her facial features, not one is striking.)
When you are considering things as a collective group (the notion of a sub-group within a larger group), use the plural:
“She had many boyfriends and love affairs in her younger years, but none were very serious.”
In your sentence, whether a singular or plural verb is used depends on the author's perspective:
If the intention is to indicate that there was no sub-group of smokers in the group; OR, that while many or all may smoke (and it is no longer PC to depict people smoking), no one has a cigarette or pipe in the picture, then "None of them are smoking."
If to indicate and emphasize that there was not a single person in that group who is shown to be a smoker, then "None of them is smoking."