Synonyms for Poser (same or very similar meaning)
WordNet sense 1 (a person who habitually pretends to be something he is not):
poseurWordNet sense 2 (a person who poses for a photographer or painter or sculptor):
modelWordNet sense 3 (a particularly difficult or baffling question or problem):
sticker, toughie, stumperFrom the ODE community, based on WordNetadd/edit