Updated answer:
You can't use negative margin in html email. To mimic this, there are 2 ways to do it, the nested tables way and the more complex rowspan way: 
<!-- The nested way -->
<table width="500" border="0" cellpadding="0" cellspacing="0" bgcolor="#CCCCCC"><!-- coloring the whole table instead of just the cells you want, will stop gaps forming on forwarding from Outlook -->
  <tr>
    <td width="200" height="80"  bgcolor="#007700">
      <table width="100%" height="80" border="0" cellpadding="0" cellspacing="0">
        <tr>
          <td height="40" bgcolor="#FFFFFF"> 
          </td>
        </tr>
        <tr>
          <td height="40" bgcolor="#CCCCCC"> 
          </td>
        </tr>
      </table>
    </td>
    <td width="100" height="80" bgcolor="#4444FF">
     <img alt="" src="" width="100" height="80" style="margin: 0; border: 0; padding: 0; display: block;">
    </td>
    <td width="200" height="80" bgcolor="#FFFFFF">
      <table width="100%" height="80" border="0" cellpadding="0" cellspacing="0">
        <tr>
          <td height="40" bgcolor="#FFFFFF"> 
          </td>
        </tr>
        <tr>
          <td height="40" bgcolor="#CCCCCC"> 
          </td>
        </tr>
      </table>
    </td>
  </tr>
  <tr>
    <td width="500" height="200" colspan="3"> 
    </td>
 </tr>
</table>
<br><br>
<!-- The fancy rowspan way -->
<table width="500" border="0" cellpadding="0" cellspacing="0" bgcolor="#CCCCCC"><!-- coloring the whole table instead of just the cells you want, will stop gaps forming on forwarding from Outlook -->
  <tr>
    <td width="200" height="40" bgcolor="#FFFFFF"> 
    </td>
    <td width="100"  height="80" rowspan="2" bgcolor="#4444FF">
     <img alt="" src="" width="100" height="80" style="margin: 0; border: 0; padding: 0; display: block;">
    </td>
    <td width="200" height="40" bgcolor="#FFFFFF"> 
    </td>
  </tr>
  <tr>
    <td width="200" height="40"> 
    </td>
    <td width="200" height="40"> 
    </td>
  </tr>
  <tr>
    <td width="500" height="200" colspan="3"> 
    </td>
 </tr>
</table>
Original answer:
For basic positioning:
Horizontally, use align="left|center|right", vertically use valign="top|middle|bottom"
Here is how to place an image center top of a table:
<table width="500" border="0" cellpadding="0" cellspacing="0" bgcolor="#CCCCCC">
  <tr>
    <td height="500" align="center" valign="top">
      <img alt="" src="" width="100" height="100" style="margin: 0; border: 0; padding: 0; display: block;">
    </td>
  </tr>
</table>