1. A security document comprising a security image area defined on a face of said document, wherein: said security image area is divided into a plurality of image element cells defining an array of image element cells across said security image area; a first type of printed tiling element is printed in selected ones of said image element cells to define a printed security image within said security image area; a second type of printed tiling element is printed in selected ones of said image element cells to define a printed complementary security image within said security image area; said image element cells define first and second diagonal axes and a cell center positioned at the intersection of said first and second diagonal axes; individual ones of said first and second printed tiling elements comprise: at least two on-axis printed units substantially aligned with said first diagonal axis, wherein said on-axis printed units are positioned on opposite sides of said cell center; and at least two pairs of off-axis printed units substantially offset from said second diagonal axis, wherein said pairs of off-axis printed units are positioned on opposite sides of said cell center.