When President Clinton moved into the Oval Office he so liked a painting by Thomas “Yellowstone” Moran that he had it hung near his desk. This weekend, the National Gallery of Art opened the first major show of the artist instrumental in getting Congress to create the first national park. His works gave many Americans their first clear impression of what the region was really like. “Looking at them is watching the myth of the West being formed,” said Nancy K. Anderson, curator of the show. The exhibit, which ties in with Yellowstone’s 125th anniversary, will be in Washington until Jan. 11, 1998, and at the Seattle Art Museum, June 11-Aug. 30.