What you are wanting is not supported out of the box. If you build a custom widget then you can use the PrintTask to export and image of the map. Here is a snippet of code I use all the time to export the map image:
        this.map.infoWindow.hide();
        var oWid = this.map.width;
        var oHgt = this.map.height;
        var printTask = new PrintTask('http://myserver/arcgis/rest/services/ExportWebMap/GPServer/Export_Web_Map');
        var template = new PrintTemplate();
        this.imgHeight = (740/oWid) * oHgt;
        template.exportOptions = {
          width: 1542,
          height: (1542/oWid) * oHgt,
          dpi: 200
        };
        template.format = "jpg";
        template.layout = "MAP_ONLY";
        template.preserveScale = false;
        template.showAttribution = false;
        var params = new PrintParameters();
        params.map = this.map;
        params.template = template;
        printTask.execute(params, lang.hitch(this, this.printResult));
...
      printResult: function(rsltURL){
        var mapImg = domConstruct.toDom('<img src="'+rsltURL.url+'" border="0" style="width:740px;height:'+this.imgHeight+'px;"/>');
        domConstruct.place(mapImg, dom.byId('mapImgDiv'), 'replace');
      },