The Collaborative International Dictionary of English v.0.48:

Portraitist \Por"trait*ist\, n.
   A portrait painter. [R.] --Hamerton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

portraitist
    n 1: a painter or drawer of portraits [syn: portraitist,
         portrait painter, portrayer, limner]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "portraitist":
   aquarellist, colorist, historical painter, landscapist, luminarist,
   luminist, marine painter, miniaturist, monochromist, oil-colorist,
   painter, pavement artist, polychromist, portrait painter,
   scene painter, scenewright, scenographer, still-life painter,
   watercolorist