var numberOfImage = 9;
var num = Math.floor(Math.random()*numberOfImage) + 1;

function changeimg(){
    document.getElementById("arm").src = "./images/arm" + num + ".jpg";
}

function writeimgtag(){
    if(num >= 20){
	document.write("<img src=\"./images/img" + num + ".png\" id=\"arm\" alt=\"logo\">");
    }else{
	document.write("<img src=\"./images/arm" + num + ".jpg\" id=\"arm\" alt=\"logo\">");
    }
}