var bgSizeX = 120,     bgSizeY = 120;
var bgPosX  = bgSizeX, bgPosY  = bgSizeY;
function mvBG() {
  var elBody = document.getElementsByTagName("body").item(0);

  elBody.style.backgroundPosition = bgPosX + "px " + bgPosY + "px";
  bgPosX -= 2; if(bgPosX < 0) bgPosX += bgSizeX;
  bgPosY -= 1; if(bgPosY < 0) bgPosY += bgSizeY;
  setTimeout("mvBG();", 100);
}