var max = 4; // num imgs - 1
var rand = Math.floor(Math.random() * (max + 1)); // random integer in [0, max]
getImg("/cgi-bin/imgserve.cgi?t=main&x="+rand, "main_img");
getImg("/cgi-bin/imgserve.cgi?t=top&x="+rand, "top_img");
getImg("/cgi-bin/imgserve.cgi?t=bot&x="+rand, "bot_img");
getImg("/cgi-bin/imgserve.cgi?t=left&x="+rand, "left_img");