In George Washington’s day, there were no photographs or cameras.
An image was often sculpted or painted. Some paintings of George Washington showed him standing behind a desk with one arm behind his back while others showed both legs and both arms.
Often painter’s fees were not based on how many people were to be painted, but by how many limbs were to be painted.
Painting both arms and both legs would cost the buyer more. Thus the expression, ‘Okay, but it’ll cost you an arm and a leg.’