function changePic(el, bg) {
	bg.src=el.src;
}

window.onload=function() {
	var one = document.getElementById('one');
	var two = document.getElementById('two');
	var three = document.getElementById('three');
	var four  = document.getElementById('four');
	var main = document.getElementById('mainImage');
	one.onclick=function(){changePic(this, main); }
	two.onclick=function(){changePic(this, main); }
	three.onclick=function(){changePic(this, main); }
	four.onclick=function(){changePic(this, main); }
}