var imgMax  = 3;
var imgRand = Math.floor(Math.random() * imgMax);

if(imgRand == 0)       document.write('<a href="tenrankai/kaisai/kaisai.html"><img src="top-images/koukoku/img1.jpg" width=152 height=347 border="0"></a>');
else if(imgRand == 1)  document.write('<a href="tenrankai/kaisai/kaisai.html"><img src="top-images/koukoku/img2.jpg" width=152 height=347 border="0"></a>');
else if(imgRand == 2)  document.write('<a href="tenrankai/kaisai/kaisai.html"><img src="top-images/koukoku/img3.jpg" width=152 height=347 border="0"></a>');