A microphotograph is a picture which is too small to see with the unaided eye. It is a microscopic photograph. So far, I have only seen one example, the microfilm of past newspapers in a library. These are photographs shrunk to microscopic scale.
A photomicrograph (or simply a micrograph) is a picture of a microscopic structure taken with the help of a microscope. These are the pictures that microscopists make with the help of a microscope.