var ar = new Array("<IMG SRC=images/pix01.jpg WIDTH=300 HEIGHT=146 border=0>", 
"<IMG SRC=images/pix02.jpg WIDTH=300 HEIGHT=146 border=0>", 
"<IMG SRC=images/pix03.jpg WIDTH=300 HEIGHT=146 border=0>", 
"<IMG SRC=images/pix04.jpg WIDTH=300 HEIGHT=146 border=0>", 
"<IMG SRC=images/pix05.jpg WIDTH=300 HEIGHT=146 border=0>");

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