var ar = new Array("<IMG SRC=html/Comstretcher3x550.jpg WIDTH=550 HEIGHT=179 BORDER=0>", 
"<IMG SRC=html/Comstretcher4x550.jpg WIDTH=550 HEIGHT=161 BORDER=0>", 
"<IMG SRC=html/Comstretcher5x550.jpg HEIGHT=177 WIDTH=550 BORDER=0>");

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