Complex sentence: In New York the dealer Macbeth established him as one of the leading portraitists of the 20th century and his over 500 portraits that continue to decorate the galleries and halls of many of America's museums and institutions.
Split into simpler sentences: In New York the dealer Macbeth established him as one of the finest portraitists of the 20th century. He painted over 500 portraits that continue to decorate the galleries and halls of many of America's museums and institutions.