var ar = new Array("<IMG SRC=html/HeaderImage1.jpg width=780 height=145 border=0>", 
"<IMG SRC=html/HeaderImage2.jpg width=780 height=145 border=0>",
"<IMG SRC=html/HeaderImage3.jpg width=780 height=145 border=0>",
"<IMG SRC=html/HeaderImage4.jpg width=780 height=145 border=0>"); 

var num = Math.floor(Math.random() * ar.length);
document.write(ar[num]);