dd = new Date();
tt = dd.getHours();

if ((tt>=0)&&(tt<=5)){document.write("<img src='./img/mainimg02.jpg'>")}
if ((tt>=6)&&(tt<=18)){document.write("<img src='./img/mainimg01.jpg'>")}
if ((tt>=19)&&(tt<=24)){document.write("<img src='./img/mainimg02.jpg'>")}