function 
checkType(upload){
    
var 
p_w_picpath = document.getElementById(upload).value;
    
var 
extStart = p_w_picpath.lastIndexOf(
"."
);
    
var 
ext = p_w_picpath.substring(extStart, p_w_picpath.length).toUpperCase();
    
if 
( ext != 
".PNG" 
&& ext != 
".GIF" 
&& ext != 
".JPG" 
&& ext != 
".JPEG"
) {
        
alert(
"图片限于png，gif，jpeg，jpg格式！"
);
        
return 
false
;
    
}
    
return 
true
;
}
function 
setImagePreview(upload,preview) {
    
if
(checkType(upload)){
        
showTips(
'result'
,
'图片预览：'
,0);
        
var 
docObj=document.getElementById(upload);
        
var 
imgObjPreview=document.getElementById(preview);
        
if
(docObj.files &&    docObj.files[0]){
            
//火狐下，直接设img属性
            
imgObjPreview.style.width = 
'160px'
;
            
imgObjPreview.style.height = 
'120px'
;
            
try
{
                
imgObjPreview.src = docObj.files[0].getAsDataURL();
            
}
catch
(e){
                
//火狐7以上版本不能用上面的getAsDataURL()方式获取，需要一下方式
                
imgObjPreview.src = window.URL.createObjectURL(docObj.files[0]);
            
}
  
  
        
}
else
{
            
//IE下，使用滤镜
            
docObj.select();
            
var 
imgSrc = document.selection.createRange().text;
            
var 
localImagId = document.getElementById(
"localImag"
);
            
//必须设置初始大小
            
localImagId.style.width = 
"160px"
;
            
localImagId.style.height = 
"120px"
;
            
//图片异常的捕捉，防止用户修改后缀来伪造图片
            
try
{
                
localImagId.style.filter=
"progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=scale)"
;
                
localImagId.filters.item(
"DXImageTransform.Microsoft.AlphaImageLoader"
).src = imgSrc;
            
}
catch
(e){
                
alert(
"您上传的图片格式不正确，请重新选择!"
);
                
return 
false
;
            
}
            
imgObjPreview.style.display = 
'block'
;
            
document.selection.empty();
        
}
        
return 
true
;
    
}
  
}

　摘自：http://www.oschina.net/code/snippet_187608_34433