preReadImg = new Array(12);

for(i=0; i<12; i++){
    preReadImg[i] = new Image();
}

j=0;
for(i=1; i<=6; i++){
    preReadImg[j].src="Image/button" + i + ".gif";j++;
    preReadImg[j].src="Image/button" + i + "a.gif";j++;
}

function changeBT1()
{
document.illustB1.src="Image/button1.gif";
}
function changeBT1a(){
document.illustB1.src="Image/button1a.gif";
}

function changeBT2()
{
document.illustB2.src="Image/button2.gif";
}
function changeBT2a()
{
document.illustB2.src="Image/button2a.gif";
}

function changeBT3()
{
document.illustB3.src="Image/button3.gif";
}
function changeBT3a()
{
document.illustB3.src="Image/button3a.gif";
}

function changeBT4()
{
document.illustB4.src="Image/button4.gif";
}
function changeBT4a()
{
document.illustB4.src="Image/button4a.gif";
}

function changeBT5()
{
document.illustB5.src="Image/button5.gif";
}
function changeBT5a()
{
document.illustB5.src="Image/button5a.gif";
}

function changeBT6()
{
document.illustB6.src="Image/button6.gif";
}
function changeBT6a()
{
document.illustB6.src="Image/button6a.gif";
}