background image not showing correctly in IE7

I've been developing an intranet for IE6 users and all has gone well.
I have just checked it out using IE7 and there is a very strange behaviour.

This is how the image looks in IE6 (resized for speed)
http://www.wanganui.govt.nz/w1.gif
and this is how it displays in IE7
http://www.wanganui.govt.nz/w2.gif
The top image (top half of fish head) is a standard <img>
The bottom half that does not show properly in IE7 is a TD background.

Here is the relevant code from the page

 <table width="955"  border="0" align="center" cellpadding="0" cellspacing="0" bgcolor="#FFFFFF">
  <tr>
    <td colspan="2" style="height: 83px; width:955"><img alt="WANDA" border="0" src="images/WandaTop2.jpg" width="955" height="83" runat="server" usemap="#isMap" /></td>
  </tr>
  <tr>

    <td style="width: 258px; height: 520px; background-image:url(/wanda/images/WandaSide.jpg); background-position: left top; background-attachment: fixed; background-repeat: no-repeat;" valign="top">
        <div style="z-index: 101; left: 8px; width: 176px; position: relative; top: 32px;">
            <br />
            <br />
       <span style="font-family:Gill Sans MT; color:White">Intranet Essentials</span><br />
       <a class="linklist" href="absentees.aspx" runat="server">Absentees</a><br />
       <a class="linklist" href="http://wdc-dvmain/Intranet/forum/login.asp?target=default.asp" runat="server">The Lounge</a><br />
       <a class="linklist" href="Tatou hui/TatouHui.doc" runat="server">Tatou Hui</a><br />
       <a class="linklist" href="socialClub/default.aspx" runat="server">Social Club</a><br />
       <span style="font-family:Gill Sans MT; color:White">External Links</span><br />
       <a class="linklist" href="http://www.sarjeant.org.nz" target="_blank">Gallery</a><br />
       <a class="linklist" href="http://www.wanganui-museum.org.nz" target="_blank">Museum</a><br />
        <a class="linklist" href="http://www.wanganuilibrary.com" target="_blank">Library</a><br />
        <a class="linklist" href="http://www.lgnz.co.nz" target="_blank"> LGNZ</a><br />
        <a class="linklist" href="http://www.royaloperahouse.co.nz" target="_blank">Opera House</a><br />
        <a class="linklist" href="http://www.wanganuiinc.co.nz" target="_blank">WINC</a><br />
        <span style="font-family:Gill Sans MT; color:White">Interactive</span><br />
         <a class="linklist" href="http://wdc-16/wdc_live/">My Payslips</a><br />
         <a class="linklist" href="#">IT Help Desk</a><br />
         <a class="linklist" href="Talk2Wanda.aspx" runat="server">Talk to Wanda</a><br />
         <a class="linklist" href="search.aspx" runat="server">Search</a><br />
        </div>
    <img alt="" src="images/spacer.gif" width="258" height="5"  runat="server" /><br />
        </td>
    <td align="left" valign="top" style="width: 689px;">
    <img src="images/wandawords.gif"  runat="server" alt="WDC Announcements" />
   <table width="685px">
  <tr>
  <td valign="top" style="width: 700px">
        <asp:contentplaceholder id="ContentPlaceHolder1" runat="server">
        </asp:contentplaceholder>

      </td>
  </tr>
  </table>