Roy Lichtenstein's estate claims copyright over the images he appropriated

Ryan sez, "My band is about to release a new record, and we decided to use a friend's painting for the cover. One day before our label was set to send away for CD & LP pressing we received an email from Shelley Lee, the Manager of Intellectual Property for the Estate of Roy Lichtenstein saying that the painting was an adapted Roy Lichtenstein image and a violation of copyright. I've been doing some research, including talking to the artist and an intellectual property lawyer, and I just don't want to feel bullied by Ms. Lee. It turns out our friend's painting was part of an appropriated art course she took in college, and she appropriated her painting from the same source image Lichtenstein used for his own work. We don't want to frantically search for an entirely different album cover at the last minute if it's not necessary. What can we do? The link I've included is to our band's website, and the main page is a posting I just put up."

A Copyright Violation??? (Thanks, Ryan!)