模拟微信上传图片（带预览，支持预览gif）

一、Html

<style type="text/css">
  #previewDiv{width:50px;height:50px;overflow:hidden;position:relative;border:1px solid;}
  #previewImg{height:100%;width:100%; position:absolute;z-index:0;left:0;}
  .img_uploader{width:50px;height:50px;z-index:1;position:absolute;left:0; filter:alpha(opacity=0);-moz-opacity:0;opacity:0;}
</style>

<div id="previewDiv">
　　<img id="previewImg" alt="" src="" /><input type="file" class="img_uploader"/>
</div>

<%--文件控件也可使用服务器控件： <asp:FileUpload ID="fileUpload" CssClass="img_uploader" runat="server" />--%>

二、JQ

//需引用jquery库
$(function () {
    $(".img_uploader").change(function () {
        PreviewImage(this, 'previewImg', 'previewDiv')
    });
});    

    function PreviewImage(fileObj, imgPreviewId, divPreviewId) {
                var allowExtention = ".jpg,.bmp,.gif,.png";//允许上传文件的后缀名document.getElementById("hfAllowPicSuffix").value;
                var extention = fileObj.value.substring(fileObj.value.lastIndexOf(".") + 1).toLowerCase();
                var browserVersion = window.navigator.userAgent.toUpperCase();
                if (allowExtention.indexOf(extention) > -1) {
                    if (fileObj.files) {//HTML5实现预览，兼容chrome、火狐7+等
                        if (window.FileReader) {
                            var reader = new FileReader();
                            reader.onload = function (e) {
                                document.getElementById(imgPreviewId).setAttribute("src", e.target.result);
                            }
                            reader.readAsDataURL(fileObj.files[0]);
                        } else if (browserVersion.indexOf("SAFARI") > -1) {
                            alert("不支持Safari6.0以下浏览器的图片预览!");
                        }
                    } else if (browserVersion.indexOf("MSIE") > -1) {
                        if (browserVersion.indexOf("MSIE 6") > -1) {//ie6
                            document.getElementById(imgPreviewId).setAttribute("src", fileObj.value);
                        } else {//ie[7-9]
                            fileObj.select();
                            if (browserVersion.indexOf("MSIE 9") > -1)
                                fileObj.blur();//不加上document.selection.createRange().text在ie9会拒绝访问
                            var newPreview = document.getElementById(divPreviewId + "New");
                            if (newPreview == null) {
                                newPreview = document.createElement("div");
                                newPreview.setAttribute("id", divPreviewId + "New");
                                newPreview.style.width = document.getElementById(imgPreviewId).width + "px";
                                newPreview.style.height = document.getElementById(imgPreviewId).height + "px";
                                newPreview.style.border = "solid 1px #d2e2e2";
                            }
                            newPreview.style.filter = "progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod='scale',src='" + document.selection.createRange().text + "')";
                            var tempDivPreview = document.getElementById(divPreviewId);
                            tempDivPreview.parentNode.insertBefore(newPreview, tempDivPreview);
                            tempDivPreview.style.display = "none";
                        }
                    } else if (browserVersion.indexOf("FIREFOX") > -1) {//firefox
                        var firefoxVersion = parseFloat(browserVersion.toLowerCase().match(/firefox\/([\d.]+)/)[1]);
                        if (firefoxVersion < 7) {//firefox7以下版本
                            document.getElementById(imgPreviewId).setAttribute("src", fileObj.files[0].getAsDataURL());
                        } else {//firefox7.0+
                            document.getElementById(imgPreviewId).setAttribute("src", window.URL.createObjectURL(fileObj.files[0]));
                        }
                    } else {
                        document.getElementById(imgPreviewId).setAttribute("src", fileObj.value);
                    }
                } else {
                    alert("仅支持" + allowExtention + "为后缀名的文件!");
                    fileObj.value = "";//清空选中文件
                    if (browserVersion.indexOf("MSIE") > -1) {
                        fileObj.select();
                        document.selection.clear();
                    }
                    fileObj.outerHTML = fileObj.outerHTML;
                }
            }

三、参考

原理：使用FileReader对象的readAsDataURL方法来读取图像文件（http://blog.okbase.net/jquery2000/archive/1296.html）

http://www.cnblogs.com/slyzly/articles/2411940.html

http://gallop-liu.iteye.com/blog/1344778  （参数previewDiv是为了兼容IE9，详情参考该链接）