microphotograph

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A photograph requiring magnification for viewing.
• noun A photograph on microfilm.

from The Century Dictionary.

• noun A photograph of any object, made so small as to require a microscope for its examination; “a microscopic photograph of a macroscopic object” (A. C. Mercer).
• noun See photomicrograph.

from the GNU version of the Collaborative International Dictionary of English.

• noun A microscopically small photograph of a picture, writing, printed page, etc.
• noun An enlarged representation of a microscopic object, produced by throwing upon a sensitive plate the magnified image of an object formed by a microscope or other suitable combination of lenses.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A photograph so reduced in size that it must be viewed through a lens or a microscope.
• noun A photograph taken through a microscope, an enlarged picture of a very small item or area; a photomicrograph.