How can display triangular pointer on the image button on click function

I have 4 images.Onclick of each image corresponding div content will be displayed.How i will get triangular pointer on image when the image is clicked. 

by using css i got the triangular image

#img {
    width: 150px;
    height: 40px;
    background-color: darkblue;
    position: relative;
    margin-left: 40px;
    border-radius: 5px;
}

#img:before {
     content: '';
    position: absolute;
    left: 42%;
    top: 100%;
    width: 0;
    height: 0;
    border-left: 20px solid transparent;
    border-right: 20px solid transparent;
    border-top: 20px solid darkblue;    
    clear: both;
}

<img src="smiley.gif" alt="Smiley face" height="42" id="img" width="42">

How i will get the triangular pointer on the button click