Setting Background Image inside a Textbox using CSS.

input[type='text']
{
    background-image:url('Images/search.gif');
    background-repeat:no-repeat;
    padding-left:20px;
}
<asp:TextBox ID="txt1" runat="server"></asp:TextBox>
<asp:TextBox ID="txt2" runat="server"></asp:TextBox>
<asp:TextBox ID="txt3" runat="server"></asp:TextBox>
<asp:TextBox ID="txt4" runat="server"></asp:TextBox>
<input type="text" />

.searchBox_1
{
    background-image:url('Images/search.gif');
    background-repeat:no-repeat; 
    padding-left:20px;
}
<asp:TextBox ID="txt" runat="server" CssClass="searchBox_1"></asp:TextBox>