How To overlap "png" image over an "jpeg" image ?

Hi friends,
I want to overlap a "png" image over an "jpeg" image. But it is not overlaping can any body tell me how to do it Here's the code i'm using
 img.s1
 {
       position:relative;
       height:100px; width:100px;
       z-index:1;
 }
 img.s2
 {
       position:inherit;
       height:100px; width:100px;
       z-index:2;
 }
<asp:ListView DataSourceID="sqldtsrcendpdt" ID="endpdtlst" runat="server">
                    <LayoutTemplate>
                        <ul class="productlist">
...