layey图片内容不居中

$('.showImg').click(function(){
	var src = $(this).prevAll('input').val();
	layer.open({
		content:'<img class="content_img" src="'+src+'">',
	});

	$("<img/>").attr("src", src).load(function() {//imgSrc是图片地址
		var realWidth = this.width;//图片宽
		var realHeight = this.height;//图片高
			
		$imgw = realWidth;
		var w_w = $(window).width(); 
		var w_h = $(window).height(); 
		var go_l = (w_w - realWidth)/2;
		var go_t = (w_h - realHeight)/2-70;
		
		if($imgw >= w_w){//当图片宽大于窗口宽时
			$imgw = w_w;
			go_l = 0;
			$('.content_img').css('width',w_w-40);
		}
		
		$('.layui-layer-dialog').width($imgw).css({
			'left':go_l,
			'top':go_t
		});

	})
	
});