Burnt Sienna by David Morrell

Burnt Sienna - Chase Malone, ex military helicopter pilot, is now living in Mexico, making his living as a landscape painter.  Derek Bellasar, international arms dealer and all around monster, wants Chase to paint his wife in two poses, one a full length nude.  Chase refuses and Bellasar uses his power to destroy Chase's life.  The CIA convinces Chase to do the portraits while spying on Bellasar.