WMS OpenLayers click on thumbnail image to show Layer

HTML

<script src="https://maps.googleapis.com/maps/api/js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/openlayers/2.13.1/OpenLayers.js"></script>
<body onload="init()">
     <h1></h1>

    <div id="map-canvas" class="smallmap"></div>
</body>

CSS

#map-canvas {
    width: 100%;
    height: 400px;
    border: 1px solid #dddddd;
    margin-top:10px;
}
.vorschau {
    width: 150px;
    height:80px;
    cursor:pointer;
    border: 1px solid #00008B;
}
#thumbnails {
    margin-bottom: 15px;
}

JavaScript

var map;


function init() {


    map = new OpenLayers.Map('map-canvas', {
        projection: 'EPSG:3857',
        layers: [new OpenLayers.Layer.Google(        "SAT", {
            type: google.maps.MapTypeId.SATELLITE,
            visibility: true,
            isBaseLayer: true
        }),
        new OpenLayers.Layer.Google(
            "SAT", {
            type: google.maps.MapTypeId.SATELLITE,
            visibility: true,
            isBaseLayer: true
        }),
        new OpenLayers.Layer.WMS("Idrografia",
            "http://geomap.reteunitaria.piemonte.it/ws/siti/aipo-01/sitiwms/wms_aipo_dati_base?", {
            layers: 'Idrografia,IdrografiaLabel',
            transparent: true
        }, {
            isBaseLayer: false,
            singleTile: true,
            queryLayers: 'Idrografia',
            visibility: true
        }),
        new OpenLayers.Layer.WMS("Limiti amministrativi",
            "http://geomap.reteunitaria.piemonte.it/ws/siti/aipo-01/sitiwms/wms_aipo_dati_base?", {
            layers: 'Regioni,Province,Comuni,ComuniLabel',
            transparent: true
        }, {
            isBaseLayer: false,
            singleTile: true,
            queryLayers: 'Comuni',
            visibility: false
        }),
        new OpenLayers.Layer.WMS("Bacini idrografici",
            "http://geomap.reteunitaria.piemonte.it/ws/siti/aipo-01/sitiwms/wms_aipo_dati_base?", {
            layers: 'Bacino,BacinoLabel',
            transparent: true
        }, {
            isBaseLayer: false,
            singleTile: true,
            visibility: false
        }),
        new OpenLayers.Layer.WMS("Carta Fiume Po 1:50.000",
            "http://geomap.reteunitaria.piemonte.it/ws/siti/aipo-01/sitiwms/wms_aipo_cartafiumepo?", {
            layers: '2011_CT50_Po',
            transparent: true
        }, {
            isBaseLayer: false,
            singleTile: true,
            visibility: false
        }),
        new OpenLayers.Layer.WMS("Competenza...