imag=new Array();
imag[0]="images/button_off.gif";
imag[1]="images/button_on.gif";
imag[2]="images/email_button_off.gif";
imag[3]="images/email_button_on.gif";

im=new Array();

for(var i=0;i<imag.length;i++)
{
  im[i]=new Image();
  im[i].src=imag[i];
}

function toggle(imgName,index)
{
  imgName.src=im[index].src;
}