I used dhtmltooltip.I do not see a change, or help my problem.
in file root.master

<head id="Head1" runat="server">
    <title></title>
    <meta name="description" content="" />
    <meta name="keywords" content="" />
   <link href="/Scripts/ddimgtooltip.css" rel="Stylesheet" type="text/css" />
    <asp:PlaceHolder id="SCRIPTS" runat="server" />
    <asp:PlaceHolder id="phAnalyticsHead" runat="server" />
</head>
in file ProductBox3.ascx

<%@ Register TagPrefix="nopCommerce" TagName="ProductPrice3" Src="~/Modules/ProductPrice3.ascx" %>
<div id="dhtmltooltip"></div>
<div class="product-item">
    <div class="picture" style="margin-top: 9px; float:left">
        <script src="../Scripts/dhtmltooltip.js" type="text/javascript"></script>
        <asp:HyperLink ID="hlImageLink" runat="server" />
    </div>
    <div style=" float:right; width: 160px;">
        <h2 class="product-title">
        <asp:HyperLink ID="hlProduct" runat="server" />
    </h2>
    <div class="prices-wrapper">
     <nopCommerce:ProductPrice3 ID="ctrlProductPrice" runat="server" ProductID='<%#Eval("ProductId") %>' />
    </div>
    </div>
</div>


in file ProductBox3.ascx.cs

private void BindData()
        {
            if (product != null)
            {
                string productURL = SEOHelper.GetProductUrl(product);
                var picture = product.DefaultPicture;
                hlProduct.NavigateUrl = productURL;
                string url = this.PictureService.GetPictureUrl(picture, this.ProductImageSize, true);
                hlProduct.Text = Server.HtmlEncode(product.LocalizedName);//<a href="#" onmouseover="Tip('<img src=\'<%#Eval("hinh")%>\'>')" onmouseout="UnTip()">
                string temp = "<img src='" + url + "' width='150px'/>,'#FFF',300";
                if (picture != null)
                {
                    hlImageLink.ImageUrl = this.PictureService.GetPictureUrl(picture, this.ProductImageSize, true);
                    //hlImageLink.ToolTip = String.Format(GetLocaleResourceString("Media.Product.ImageLinkTitleFormat"), product.LocalizedName);
                    hlImageLink.Text = String.Format(GetLocaleResourceString("Media.Product.ImageAlternateTextFormat"), product.LocalizedName);
                    hlImageLink.Attributes["onMouseOver"] = "ddrivetip('" + temp + "')";
                    hlImageLink.Attributes["onMouseOut"] = "hideddrivetip()";
                }
                else
                {
                    hlImageLink.ImageUrl = this.PictureService.GetDefaultPictureUrl(this.ProductImageSize);
                    //hlImageLink.ToolTip = String.Format(GetLocaleResourceString("Media.Product.ImageLinkTitleFormat"), product.LocalizedName);
                    hlImageLink.Text = String.Format(GetLocaleResourceString("Media.Product.ImageAlternateTextFormat"), product.LocalizedName);
                    hlImageLink.Attributes["onMouseOver"] = "ddrivetip('" + temp + "')";
                    hlImageLink.Attributes["onMouseOut"] = "hideddrivetip()";
                }
                hlImageLink.NavigateUrl = productURL;
                var productVariantCollection = product.ProductVariants;
                if (productVariantCollection.Count > 0)
                {
                    if (!product.HasMultipleVariants)
                    {
                        var productVariant = productVariantCollection[0];
                        if (!this.SettingManager.GetSettingValueBoolean("Common.HidePricesForNonRegistered") ||
                            (NopContext.Current.User != null &&
                            !NopContext.Current.User.IsGuest))
                        {
                            //nothing
                        }
                    }
                }
            }
        }

i used dhtmltooltip:
http://www.dynamicdrive.com/dynamicindex5/dhtmltooltip.htm