JSFiddle - React, Tailwind, and code Playground

HTML

<div id="weisseFlaeche"></div>
<div id="grp">
    <img src="http://upload.wikimedia.org/wikipedia/commons/3/3d/FuBK-Testbild.png" onclick="oeffneFenster()" width="816" height="616" />
</div>

CSS

#weisseFlaeche {
    left:0px;
    top:0px;
    width:100%;
    height:100%;
    position:fixed;
    background:white;
    opacity:0.6;
    display: none;
}

JavaScript

function oeffneFenster(){
    document.getElementById('weisseFlaeche').style.display = 'block';
}