There are no configuration options (or API methods) which would allow you to toggle the thumbnails without the main image resizing dynamically to fill the available space.
However, you could try the following which places a custom 'Toggle Thumbnails' button on the web page and uses jQuery to toggle the .jb-classifier-thumb-area (thumbnail) container.
You would need to include jQuery in your web page for this solution to work.
You could do this with pure JavaScript but the function would not be quite as concise.
Also, you could attach the click handler to whatever element on your web page you like (it does not have to be a custom <input> tag like in the example).

<!--START JUICEBOX EMBED-->
<script src="jquery-1.11.1.min.js"></script>
<script src="jbcore/juicebox.js"></script>
<script>
    new juicebox({
        containerId: 'juicebox-container',
        galleryWidth: '600px',
        galleryHeight: '400px'
    });
    $(document).ready(function() {
        $('#toggle-thumbnails').click(function() {
              $('.jb-classifier-thumb-area').toggle();
        });
    });
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->
<div>
    <input id="toggle-thumbnails" type="button" value="Toggle Thumbnails" />
</div>