Source : Webster's Revised Unabridged Dictionary (1913)

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}.

   2. A microscopically small photograph of an object.

Source : WordNet®

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