Source : Webster's Revised Unabridged Dictionary (1913)

Photo \Pho"to\, n.; pl. {Photos}.
   A contraction of {Photograph}. [Colloq.]

Source : WordNet®

photo
     n : a picture of a person or scene in the form of a print or
         transparent slide; recorded by a camera on
         light-sensitive material [syn: {photograph}, {exposure},
         {pic}]