This way
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<style type="text/css">
.search
{
background-image: url('find.png');
background-repeat: no-repeat;
padding-left: 20px;
border:1px solid #ccc;
}
</style>
</head>
<body>
<form id="form1" runat="server">
<asp:TextBox ID="TextBox1" runat="server" CssClass="search"></asp:TextBox>
</form>
</body>
</html>