Gridview CSS background-image IE8 不工作

我在母板页里写了下面这段style：
  <style type="text/css">  
  .ListHead2
  {
  background-color: #E0E3E8;
  height: 20px;
  color: #000000;
  font-family: Tahoma, Arial, Verdana, Tahoma, Arial;
  font-size: 8.5pt;
  font-weight: normal;
  text-align: left;
  cursor: default;
  font-weight: normal;
  background-image:url(../images/img_mnusep.gif);
  background-repeat: no-repeat;
  padding-left: 8px;
  padding-right: 2px;
  border : 0px;  
  border-bottom : solid 1px #A0A0A4;  
  }
  </style> 

然后我在内容页里有这样一个gridview：
<asp:GridView ID="myGridView" runat="server" AutoGenerateColumns="False" HeaderStyle-CssClass="ListHead2"
  CssClass="List" BorderWidth="0px" DataKeyNames="criterionId" Width="100%">
  <Columns>
  <asp:TemplateField>