I'm working with OpenLayers 3 on creating a webgis I want to add a caption (png image) of a map via a button, as I demonstraded.

When someone clicks the button I would like the caption image to appear inside the map.

<button id="testButton" onclick="legend()" class="fas fa-list-ul"> <img src="http://datageo.ambiente.sp.gov.br/geoserver/datageo/wms?REQUEST=GetLegendGraphic&VERSION=1.1.0&FORMAT=image/png&WIDTH=20&HEIGHT=20&LAYER=datageo:DensidadeDemografica2016&bbox=-124.73142200000001,24.955967,-66.969849,49.371735&srcwidth=768&srcheight=330&srs=EPSG:4326&legend_options=countMatched:true;fontAntiAliasing:true"> </button>

The caption is generated through a WMS request from OpenLayes 3 itself, by REQUEST = GetLegendGraphic.

I tested this alternative, however, without success. I've added my layers inside an array to make the view call from them, as you can see.

Can you help me?

        var layersArray = [];
        var map;

        function layer () {

            var osm = new ol.layer.Tile({
                preload: Infinity,
                source: new ol.source.OSM()

            var layer =  new ol.layer.Image({
                        source: new ol.source.ImageWMS({
                            url: 'http://datageo.ambiente.sp.gov.br/geoserver/datageo/wms',
                            params: {'layers':'datageo:DensidadeDemografica2016', 'TILED':true},
                            ratio: 1,
                            serverType: 'geoserver'

            var layerUm =  new ol.layer.Image({
                source: new ol.source.ImageWMS({
                    url: 'http://datageo.ambiente.sp.gov.br/geoserver/datageo/wms',
                    params: {'layers':'datageo:DensidadeDemografica2015', 'TILED':true},
                    ratio: 1,
                    serverType: 'geoserver'

            layersArray.push(osm); //0
            layersArray.push(layer); //1
            layersArray.push(layerUm); //2

             map = new ol.Map({
                    target: 'map',
                    controls: ol.control.defaults({attribution:false}).extend([
                        new ol.control.ScaleLine(),
                        new ol.control.ZoomSlider()
                    renderer: 'canvas',
                    layers: layersArray,
                    view: new ol.View({
                    center: [-23.6526023,-46.8906052],
                        zoom: 4

        // funcao para aparecer e desaparecer a legenda quando clicar no botao
        function legend() {
            layersArray.events.register("visibilitychanged",layersArray, function() {
            if (layersArray.getVisible() === true) {
                } else {

       //funcao para ligar e desligar uma layer num checkbox
        function layerswitch(evt){


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