Die Softly (Vintage Paperback) by Christopher Pike

He might have photographed a murder.

Herb just wanted to photograph the cheerleaders in the school showers. He planted his camera high in the corner where no one could see it, and rigged it to a special homemade timer on Thursday night. 

But a girl dies Friday afternoon -- and when Herb collects his roll of film, he develops a picture that shows a shadowy figure approaching her in the showers. It makes Herb wonder if she truly died in a car accident. 

Unfortunately for Herb, he doesn't have to wonder if the murderer knows about the picture.