From Montaigne 1.2 De la Tristesse (On Sadness):
…. the account goes on to tell us that Cambyses asked [Psammenitus] why he had remained unmoved by the fate of his son and daughter yet showed such emotion at the death of his friend. ‘Only the last of these misfortunes can be expressed by tears’, he replied; ‘the first two are way beyond any means of expression.’
That may explain the solution adopted by a painter in antiquity. He had to portray the grief shown on the faces of the people who were present when Iphigenia was sacrificed, giving each of them the degree of sorrow appropriate to his feelings of involvement in the death of that fair and innocent young woman. By the time he came to portray the father of Iphigenia he had exhausted all the resources of his art, so he painted him with his face veiled over, as though no countenance could display a grief so intense.
… elle adjouste que Cambises s’enquerant à Psammenitus, pourquoy ne s’estant esmeu au malheur de son fils et de sa fille, il portoit si impatiemment celuy d’un de ses amis: C’est, respondit-il, que ce seul dernier desplaisir se peut signifier par larmes, les deux premiers surpassans de bien loin tout moyen de se pouvoir exprimer. A l’aventure reviendroit à ce propos l’invention de cet ancien peintre, lequel, ayant à representer au sacrifice de Iphigenia le dueil des assistans, selon les degrez de l’interest que chacun apportoit à la mort de cette belle fille innocente, ayant espuisé les derniers efforts de son art, quand se vint au pere de la fille, il le peignit le visage couvert, comme si nulle contenance ne pouvoit representer ce degré de dueil.
The painter is Timanthes (4th century B.C) and the painting described is mentioned several times in Roman sources.
Pliny the Elder (35.74) says:
To return to Timanthes—he had a very high degree of genius. Orators have sung the praises of his Iphigenia, who stands at the altar awaiting her doom; the artist has shown all present full of sorrow, and especially her uncle, and has exhausted all the indications of grief, yet has veiled the countenance of her father himself. whom he was unable adequately to portray. There are also other examples of his genius, for instance a quite small panel of a Sleeping Cyclops, whose gigantic stature he aimed at representing even on that scale by painting at his side some Satyrs measuring the size of his thumb with a wand. Indeed Timanthes is the only artist in whose works more is always implied than is depicted, and whose execution, though consummate, is always surpassed by his genius.
Nam Timanthis vel plurimum adfuit ingenii. eius enim est Iphigenia oratorum laudibus celebrata, qua stante ad aras peritura cum maestos pinxisset omnes praecipueque patruum et tristitiae omnem imaginem consumpsisset, patris ipsius voltum velavit, quem digne non poterat ostendere. sunt et alia ingenii eius exempla, veluti Cyclops dormiens in parvola tabella, cuius et sic magnitudinem exprimere cupiens pinxit iuxta Satyros thyrso pollicem eius metientes. atque in unius huius operibus intelligitur plus semper quam pingitur et, cum sit ars summa, ingenium tamen ultra artem est.
And Cicero (Orator 22):
the poet avoids impropriety as the greatest fault which he can commit; he errs also if he puts the speech of a good man in the mouth of a villain, or that of a wise man in the mouth of a fool; so also the painter in portraying the sacrifice of Iphigenia, after representing Calchas as sad, Ulysses as still more so, Menelaus as in grief, felt that Agamemnon’s head must be veiled
quod si poeta fugit ut maximum vitium qui peccat etiam, cum probam orationem affingit improbo stultove sapientis; si denique pictor ille vidit, cum immolanda Iphigenia tristis Calchas esset, tristior Ulixes, maereret Menelaus, obvolvendum caput Agamemnonis esse, quoniam summum illum luctum penicillo non posset imitari;
The below fresco from Pompeii is sometimes thought to be a Roman copy of Timanthes’ work – though it would seem necessary to take copy in a somewhat loose sense. Iphigenia is not standing at an altar as Pliny states and, Agamemnon excepted, I can’t match any of the figures with the emotions he or Cicero describe as so manifestly present.