We actually found some nice instruction for a java script here:
https://www.tutorialrepublic.com/faq/how-to-increase-and-decrease-image-size-using-javascript.php
and implemented it this way - looks ok?
<script type="text/javascript">
<!--
function zoomin(){
var myImg = document.getElementById("imagetest");
var currWidth = myImg.clientWidth;
if(currWidth == 700){
alert("Maximum zoom-in level reached.");
document.getElementById("DI04_01").value = (currWidth);
} else{
myImg.style.width = (currWidth + 20) + "px";
document.getElementById("DI04_01").value = (currWidth +20);
document.getElementById("PC01_01").value = screen.width;
document.getElementById("PC01_02").value = screen.height;
}
}
function zoomout(){
var myImg = document.getElementById("imagetest");
var currWidth = myImg.clientWidth;
if(currWidth == 50){
alert("Maximum zoom-out level reached.");
document.getElementById("DI04_01").value = (currWidth);
} else{
myImg.style.width = (currWidth - 20) + "px";
document.getElementById("DI04_01").value = (currWidth -20);
document.getElementById("PC01_01").value = screen.width;
document.getElementById("PC01_02").value = screen.height; }
}
// -->
</script>
<h2> How close do you want to be to this person? <br>
Please adjust using the zoom buttons:
</h2>
<p>
<button type="button" onclick="zoomin()">Zoom In</button>
<button type="button" onclick="zoomout()">Zoom Out</button>
</p>
<img src="imagetest.jpg" id="imagetest" width="250" alt="testimage">