Do it using Print task
var printTaskURL = "<URL of Print Service>";
var templatePrint = new esri.tasks.PrintTemplate();
templatePrint.format = "JPG";
templatePrint.layout = "MAP_ONLY";
templatePrint.exportOptions = {
width: map.width,
height: map.height,
dpi: 96
};
var params = new esri.tasks.PrintParameters();
params.map = map;
params.preserveScale = false;
params.currentExtent = map.extent;
params.template = templatePrint;
var printTask = new esri.tasks.PrintTask(printTaskURL);
dojo.connect(printTask, 'onComplete', function (result) {
window.open(result.url, "_blank");
});
dojo.connect(printTask, 'onError', function (error) {
alert(error);
})
printTask.execute(params);