var Array1 = new CreateArray('logo1', 'logo2', 'logo3', 'logo4');

function CreateArray(){
	this.length = CreateArray.arguments.length
	for (var i = 0; i < this.length; i++)
	this[i + 1] = CreateArray.arguments[i]
}

function randNum (num) {
var rnd1 = Math.round( (num-1) * Math.random() + 1 )
return rnd1;
}

document.write ("<img class=\"logo\" src=\"images/"+Array1[randNum(Array1.length)]+".png\" alt=\"logo\">");