function randomBGImage(){
	var backgr1 = "url('images/bgs/picture_bg4.jpg')"
	var backgr2 = "url('images/bgs/picture_bg2.jpg')"
	var backgr3 = "url('images/bgs/picture_bg3.jpg')"
	
	var cur=Math.round(6*Math.random())
	if (cur<=1)
	backgr=backgr1
	else if (cur<=4)
	backgr=backgr2
	else
	backgr=backgr3
	document.getElementById('mainPicture').style.backgroundImage = backgr ;

};