The Collaborative International Dictionary of English v.0.48:

Photomicrograph \Pho`to*mi"cro*graph\
   (f[=o]`t[-o]*m[imac]"kr[-o]*gr[.a]f), n. [Photo- + micro +
   -graph.]
   1. An enlarged or macroscopic photograph of a microscopic
      object. See Microphotograph.
      [1913 Webster]

   2. A microscopically small photograph of an object.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Micrograph \Mi"cro*graph\, n. [See Micrography.]
   1. An instrument for executing minute writing or engraving.
      [1913 Webster]

   2. a graphic image, such as a photograph or drawing,
      representing an object as seen with a microscope, usually
      much enlarged as compared with the original object. A
      photograph of a microscopic image is also called a
      photomicrograph or microphotograph.
      [PJC]

WordNet (r) 3.0 (2006):

photomicrograph
    n 1: a photograph taken with the help of a microscope