function preload(i)
{
   img = new Image();
   img.src = i;
}

function animate()
{
   document.images.jumper.src='images/jump.gif';
}

function unanimate()
{
   document.images.jumper.src='images/jump_static.gif';
}

preload('images/jump.gif');
preload('images/jump_static.gif');