var randomnumber=Math.floor(Math.random()*3);

var backImage = new Array();
backImage[0] = "url('./gfx/backgrounds/2.jpg') repeat fixed 50% 0 #202020";
backImage[1] = "url('./gfx/backgrounds/1.jpg') repeat fixed 50% 0 #202020";
backImage[2] = "url('./gfx/backgrounds/2.jpg') repeat fixed 50% 0 #202020";
backImage[3] = "url('./gfx/backgrounds/1.jpg') repeat fixed 50% 0 #202020";

function changeBGImage(){
  document.body.style.background = backImage[randomnumber];
  randomnumber=Math.floor(Math.random()*3);
}