openlayers3中如何叠加png图片

		image = new ol.layer.Image({
	            source: new ol.source.ImageStatic({
	               url: "img/china2.png",
	               imageExtent: extent
	            })
	        })

var extent = [63.9796331669, 14.7451916711, 140.1812559169, 55.4673388687];

	<script type="text/javascript">
		var map, image;
		function init(){
			var bounds = [72.985, 17.006, 134.416, 54.815];
			var extent = [63.9796331669, 14.7451916711, 140.1812559169, 55.4673388687];
			image = new ol.layer.Image({
	            source: new ol.source.ImageStatic({
	               url: "img/china2.png",
	               imageExtent: extent
	            })
	        })
			
			var projection = new ol.proj.Projection({
				code: 'EPSG:4326',
				units: 'degrees'
			});
			var vec_c = getTdtLayer("vec_w");
			var province = new ol.layer.Image({
		        source: new ol.source.ImageWMS({
			          ratio: 1,
			          url: 'http://10.16.48.185:8086/geoserver/my_test/wms',
			          params: {
			          		'FORMAT': 'image/png',
			                'VERSION': '1.1.1',
			                STYLES: '',
			                LAYERS: 'my_test:province_line',
			          }
		        })
		    });
			map = new ol.Map({
				controls: ol.control.defaults({
					attribution: false
				}),
				target: 'map',
				layers: [vec_c,image,province],
				view: new ol.View({
					projection: projection,
					minZoom:4,
					maxZoom:18
				})
			});
			map.getView().fit(bounds, map.getSize());
		}

		function getTdtLayer(lyr){
			var url = "http://t0.tianditu.com/DataServer?T="+lyr+"&X={x}&Y={y}&L={z}";
    		var layer = new ol.layer.Tile({
				source: new ol.source.XYZ({
			        url:url
			    })
			});
			return layer;
		}
	</script>