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>