A number of explanations might apply to this puzzling photograph. The first is that an image of Pozner was mistakenly included in the vigil by someone who turned up his picture while performing an online search using a term such as "December school massacre" to find photos of those killed in Peshawar and and did not recognize his face (because he had dark hair and facial features not unlike those of many of the victims in Peshawar). Another is that his image was deliberately included by mourners as an homage in order to publicly link multiple incidents of young, innocent victims' losing their lives in senseless school shootings. A third is that a massive worldwide cabal colluded to perpetrate or falsify the murders of more than 150 students and teachers collectively spanning the globe from Connecticut to Pakistan, and the puppeteers of this conspiracy were so inept that they failed the basic task of obtaining novel images to assign to each of the victims of the violent acts.
In fact, the second explanation offered above appears to be what happened: On the day of the Peshawar incident a Pakistani woman created a collage of photographs of young people killed in school shootings and posted it to Facebook with the caption "They Went to School and Never Came Back." Because the Peshawar shooting occurred very close to the anniversary of the Sandy Hook massacre, she included a picture of a child from the latter event, namely Noah Pozner, along with pictures of Peshawar victims