Will your table always only have 2 rows? Such as:
<table>
<tr>
<td></td>
<td></td>
</tr>
</table>
If so, a simple but not overly elegant solution would be to split your background image into two images, and apply a CSS class to the left and right column, applying half of the arrow to the right side of the left column, and to the left side of the right column:
<table>
<tr>
<td class="left"></td>
<td class="right"></td>
</tr>
</table>
Your CSS could then be similar to:
td.left
{
background: #ffffff url(../PathToLeftBackground.png) top right;
}
td.right
{
background: #fffff url(../PathToRightBackground.png) top left;
}
You could also use a sprite image where you use 1 image and position it differently for the two backgrounds.
I realize it's probably not the most ideal solution but it would at least fix your issue and get your project moving. I sometimes use simple solutions such as this in order to make forward progress, and then revisit the problem to make it more efficient later.
Hope this helps!