Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is the horror film that ruined the career of Michael Powell, the director whose previous collaborations with Emeric Pressburger created many British film classics, such as The Red Shoes, Black Narcissus, and Stairway to Heaven.
Mark, the Peeping Tom of the title, works in a film studio and sells cheesecake photos to porn shops in his spare time. Another of his hobbies is making a documentary film showing the expressions of terror on women's faces as he kills them. He's the quiet boy-next-door psycho-killer who everybody always describes to the news reporters as the last person they would ever suspect of being capable of such horrible crimes. When his next-door neighbor Helen pays a visit, he shows her an old black-and-white movie made by his father when Mark was a young boy. He explains that his father, a brilliant biologist, was interested in the reactions of the nervous system to fear, especially in children. He filmed Mark continually as he was growing up because he wanted a record of a growning child. In the home movie, we see Mark sleeping in bed as a lizard is dropped on his bed. He wakes up terrified as the lizard crawls around the bedcovers then towards him. (I probably would have been thrilled if lizards dropped on me when I was young, but it's probably more terrifying for a young English boy since they hardly have any lizards there.) The movie also contains footage of Mark and his father spying on a couple kissing in a park. These things are obviously shown to give us the psychological background for his psycopathic behavior - his father's "research." If the root of real mental problems were as simple as they are shown here, and as they are usually depicted in movies, then we'd all be perfectly sane and all the anti-depressant drug manufacturers and the rest of the mental-health-industrial-complex would be bankrupt. Wouldn't that be nice.
The lizard appears to be a type of Agamid lizard, but that's the best I can do on the ID. It's not a species not native to England, where the movie was made.