num=Math.floor(Math.random()*4)
  if(num==0) {document.write('<img border="0" src="graphics/main_pic1.jpg" width="350" height="165">')}
  if(num==1) {document.write('<img border="0" src="graphics/main_pic2.jpg" width="350" height="165">')}
  if(num==2) {document.write('<img border="0" src="graphics/main_pic3.jpg" width="350" height="165">')}
  if(num==3) {document.write('<img border="0" src="graphics/main_pic4.jpg" width="350" height="165">')}