代码如下，请自行修改并应用

private void GridView1_ItemDataBound(object sender, System.Web.UI.WebControls.DataGridItemEventArgs e)
{
    if(e.Item.ItemType == System.Web.UI.WebControls.ListItemType.Header)
    {
        e.Item.Attributes.Add("style", "background-image:url('background.gif')");
    }
}
GridView：

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.Header)
    {
        e.Row.Attributes.Add("style", "background-image:url('background.gif')");
    }
}