@*text-align:center;水平居中

vertical-align: middle;display: table-cell;垂直居中*@

<div id="img-dialog" style="text-align:center;vertical-align: middle;display: table-cell;padding:5px;">

　　<img id="showImg" />

</div>

<a href="javascript:void(0)" id="btnShow" onclick="showImga()"></a>

//图片预览

function showImga() {

    //显示图片

    $("#showImg").attr("src", "../UploadFile/Images/图片.png");

    var img = new Image();

    img.src = $('#showImg').attr("src");

    var w = img.width; //获取图片实际宽度

    var h = img.height;

    var objImg = $("#showImg")[0];

    var maxWidth = 500;

    var maxHeight = 500;

    autoResizable(w, h, maxWidth, maxHeight, objImg);

    $('#img-dialog').dialog({

        title: '图片预览',

        width: maxWidth,

        height: maxHeight,

        resizable: true,    //设置窗体大小可拖动

        onResize: function (width, height) {    //窗体大小发生改变时触发

            //这里为了显示完整图片，需减去easyui本身弹框dialog边框所占高度

            autoResizable(w, h, width - 25, height - 35, objImg);

        }

    });

}

//自动调整图片大小

//参数：图片宽度，图片高度，窗体宽度，窗体高度，图片对象

function autoResizable(w, h, maxWidth, maxHeight, objImg) {

    var hRatio;

    var wRatio;

    var Ratio = 1;

    wRatio = maxWidth / w;

    hRatio = maxHeight / h;

    if (maxWidth == 0 && maxHeight == 0) {

        Ratio = 1;

    } else if (maxWidth == 0) {

        if (hRatio < 1) Ratio = hRatio;

    } else if (maxHeight == 0) {

        if (wRatio < 1) Ratio = wRatio;

    } else if (wRatio < 1 || hRatio < 1) {

        Ratio = (wRatio <= hRatio ? wRatio : hRatio);

    }

    if (Ratio < 1) {

        w = w * Ratio;

        h = h * Ratio;

    }

    objImg.style.height = Math.round(h) + "px";

    objImg.style.width = Math.round(w) + "px";

}