var imgMax  = 3; //セットする画?の??。
var imgRand = Math.floor(Math.random() * imgMax);

if(imgRand == 0)       document.write('<a href="#"><img src="images/start1.jpg" width="697" height="233" border="0"></a>');
else if(imgRand == 1)  document.write('<a href="#"><img src="images/start2.jpg" alt="" width="697" height="233" border="0"></a>');
else if(imgRand == 2)  document.write('<a href="#"><img src="images/start3.jpg" alt="" width="697" height="233" border="0"></a>');
else if(imgRand == 3)  document.write('<a href="#"><img src="images/start1.jpg" alt="" width="697" height="233" border="0"></a>');