It happens sometimes that when people take pictures they see some faces or something that slightly resembles a human silhouette etc. on them later. And of course, the first thing they say – it’s a ghost! Well, I’m not an expert, but as for me, I think that it is just the coincidental effect of darkness and light which combined together gives this optical illusion. Plus pareidolia (e.g. seeing images of animals or faces in clouds) plays a big role in it.