It seems pretty obvious: Individual A buys a piece of art for, lets says $10,000. A couple of years goes by, and the individual puts it up for sale at an art gallery. Individual B's company buys the painting for $100,000 - Voila, $90,000 laundered completely legally as private sale of art isn't taxed.
All that is required is a small group of friends with companies they control. Why the art gallery - to make it seem more legit that they are facilitating the sale.