How to add an image after clicking a button using Javascript?

I want to add an image By clicking the button ("click here to discover"). How can I do it? this is the code:

<!DOCTYPE html>
    <title>Benefit of the month</title> 
    <h1>Benefit of the month</h1>
        p { background-color:rgb(137, 204, 152); }
        h1 { background-color:rgb(244, 143, 170); }
        h1 { color:rgb(74, 35, 13);font-family: sans-serif; }
    <meta charset="utf-8" /> 
<script type="text/javascript">
    function textchange(id="benefit"){
        document.getElementById('b').innerHTML = 'free food bag!';
    <p>And the benefit of the month is... <span id="b">...</span></p>
    <input type="button" onclick="textchange()" value="click here to discover" />