In Jean-Honoré Fragonard’s “fantasy portraits,” his congenial sitters are caught mid-stream in a range of pleasurable, intimate activities. The spontaneity and speed of his performance are palpable: hues are blended wet on wet; brush strokes retain their traces; the tip of his brush inscribes zig-zag scribbles deep into the impasto of ruffs, collerettes, and sleeves.