Random image with probability with HTML/Javascript

I have a question about generating a random image with different probabilities. I've googled, I found one comparable question (Is it possible to add weight to images in a generator?) but this code doesn't work for me.

I'm practicing on a free website now, so I can't use PHP. I only can use HTML. Someone told me, if I can use html, I can use Javascript too.

Can anyone help me? I'm trying this code now, but it doesn't work:

<script language="JavaScript">

function random_imglink(){
var myimages=new Array();
myimages[1]="http://ift.tt/1Wf8h0J"
myimages[2]="http://ift.tt/1NaVftM"
myimages[3]="http://ift.tt/1Wf8h0N"
myimages[4]="http://ift.tt/1NaVhBT"

var probabilities=new Array();
probabilities[1]=0.4;  
probabilities[2]=0.3;  
probabilities[3]=0.2; 
probabilities[4]=0.1;

function getImage() {
    var rand = Math.random();
    var probabilitiy_sum = 0;
    for(var i=0; i < probabilities.length; i++) {
        probabilitiy_sum += probabilities[i];
        if(rand <= probabilitiy_sum ) {
            return myimages[i];
        }
    }
    return myimages[myimages.length];
}

</script>
</a></p>

Thanks!