(PHOTO TALK) If you don't know about the Henry's distinguished Monsen Photography Collection, then it's time to catch up, and not just because, as one snooty art museum director once sniffed at me about an unrelated matter, "It's very well-known." How and why did Joseph Monsen get the goods? Ask him yourself in his talk accompanying the show Monsen 75 at 75. (Henry Art Gallery, 15th Ave NE and NE 41st St, 543-2280. 7 pm, free.)