var ar = new Array("<IMG SRC=html/AirAmb2x550.jpg WIDTH=550 HEIGHT=190 BORDER=0>", 
"<IMG SRC=html/AirAmb1x550.jpg WIDTH=550 HEIGHT=163 BORDER=0>", 
"<IMG SRC=html/AirAmb3x550.jpg WIDTH=550 HEIGHT=157 BORDER=0>", 
"<IMG SRC=html/AirAmb4x550.jpg HEIGHT=183 WIDTH=550 BORDER=0>");

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