Portraitists are generally required to show the "impossible". Thei own psycology is tests by not showing their own vision about it, but making us use the ours to comprehend their work: Our point of view about what's being portrayed. As I learnt in english translation in University, it's better to show the meaning of a sentence rather translating a text literally. The meaning of that text is that portraits shouldn't be subjectives about their work (they shouldn't impose their ideas to make their "master work" to mean only what they think or feel), but they should be objective and make us think (taking in count our own point of view and vision) to give our own meaning to a portrait they do.< "se pone a prueba" means "it's being tested" (something or also somebody). I hope I'd helped you! :D

Poner a prueba a alguien es comprobar si la persona es capaz o no de hacer aquéllo que se le pide.