var ic = 4;
var mainArray = new Array(ic);
        
mainArray[0] = 'images/main_01.jpg';
mainArray[1] = 'images/main_02.jpg';
mainArray[2] = 'images/main_03.jpg';
mainArray[3] = 'images/main_04.jpg';

function pickRandom(range) {
	if (Math.random)
		return Math.round(Math.random() * (range-1));
	else {
		var now = new Date();
		return (now.getTime() / 1000) % range;
	}
}

var choice = pickRandom(ic);