1. A security document comprising a document face and a security image area defined on said document face, wherein: said security image area comprises a plurality of security image blocks; each of said security image blocks is occupied by a set of security image elements and a set of complementary security image elements; said security image elements occupy a security image element portion of each of said security image blocks; said complementary security image elements occupy a complementary security image element portion of each of said security image blocks; said security image elements and said complementary security image elements in respective security image blocks are arranged such that said security image element portion and said complementary security image element portion occupy respective first and second percentages of each of said security image blocks to define respective characteristic relative occupation values for each security image block; at least one of said plurality of security image blocks defines a first security image and at least one of said plurality of security image blocks defines a second security image such that said first security image is dimensionally distinct from said second security image; and said respective characteristic relative occupation values of said first security image and said second security image are substantially equivalent.