var mainMax = 3;
var mainRandom = Math.floor(Math.random() * mainMax);
 if(mainRandom == 0) document.write('<img src=main_img/1.jpg width=480 height=192 alt=ああいい女>');
if(mainRandom == 1) document.write('<img src=main_img/2.jpg width=480 height=192 alt=ああいい女>');
else if(mainRandom == 2) document.write('<img src=main_img/3.jpg width=480 height=192 alt=ああいい女>');