Takao Nishino was only 16 when the Beatles performed at Budakon. He wanted to go, but he was a student in Osaka at the time and he didn't have the money. "Three of my friends went and they brought me a program," he recently told me. His favorite song back then was "She Loves You," because it was the first Beatles tune he ever heard and it simply blew his mind. Later it was "In My Life"—"I loved the melody. And as a 20-year-old, I felt I could identify with the lyrics"—and then "A Day in the Life," which stands out to Nishino because it bears both John's and Paul's creative stamp.

Nishino went on to become the owner of a thriving record-store business, and in 1989 he paid about $280,000 (roughly $500,000 today) for "Images of a Woman." It was a large sum, but Nishino says it wasn't a worrisome outlay—not enough to bother telling his wife, anyway. "I had a lot of money," he says, laughing. "It was the bubble."

Besides, the painting was worth it: "I'd never seen anything like it, especially all those psychedelic colors." He believes it reveals an uncanny unity: a cohesive image composed by four individuals. "In that sense," he says, "I suppose it beats 'A Day in the Life' as a truly collaborative work." His favorite part? "Ringo's corner is just beautiful. George's is weird. I can't really understand it."

For three years, "Images of a Woman" hung on the wall in Nishino's living room. But after acquiring a number of large Warhol lithographs, there just wasn't enough wall space for everything. "When I bought it, I had also noticed that it was not well kept," Nishino says. "Over the long term, the heat and humidity of Japan's summer was going to be detrimental, so I bought this $5,000 humidity-controlled frame." Noticing that the family dog, Taro, often escaped the heat by lying under the bed, he figured that would be a good place to store the boxed-up painting.

Over the years, Nishino would lend it out now and then to a Japanese television station for a story, but he would always do so under a pseudonym. "I never intended to keep it such a secret, but I was worried about security," he says. In 2002, he tried to sell it on eBay, but the deal fell through when it turned out that the buyer was using a stolen ID.

Now, 50 years after the band was first formed, Nishino is parting with the painting for real at a September 14 auction. "Originally, I thought it might be best kept as a piece of Japan's cultural heritage; it has never left Japanese soil in 46 years. But the Beatles phenomenon was and remains a global one," he says, so it's up to fans—and the marketplace—to determine where it ends up next.