Text overlays over image using relative positioning

You want position: absolute and the container to be relative:

<td width="10%" style="position: relative;">
    <img src="tempballoon.png" alt="balloon"  style="z-index: -1"/>
    <div style="position:absolute;left:0px;top:0px;font-size: 32px;display: none">
      Test
    </div>
</td>

Why not set a div inside the TD set the image to the background of the div and the drop your text in the div?

<td width="10%">
 <div style="background: transparent url("tempballoon.png") no-repeat left top; font-size: 32px;width: 100%; height: height:[height of image]">
  Test
 </div>
</td>