Thrift stores can be great places to find budget-friendly large frames. But a trip to a Savers location in Manchester, New Hampshire, in 2017 yielded much more for one bargain hunter, with a $4 painting now estimated to be worth as much as $250,000. According to Bonhams auction house, the artwork was actually an oil panel illustration by the artist Newell Convers Wyeth, father of the more-famous Andrew Wyeth. The painting was one of four cover illustrations for a 1939 edition of the Helen Hunt Jackson novel Ramona. The dramatic scene portrays the novel’s title character—a half-Scottish, half–Native American orphan...