You can use css style so background image will be adjusted all the time with table size.
There is code :
.tableBckImg
{
background:url(http://www.placehold.it/300x300);
background-repeat:no-repeat;
background-size:100% 100%;
}
<table cellspacing="0" cellpadding="0" border="0" class="tableBckImg">
<tr>
<td width="50" align="center">1</td>
<td width="50" align="center">2</td>
<td width="50" align="center">3</td>
</tr>
<tr>
<td width="50" align="center">4</td>
<td width="50" align="center">5</td>
<td width="50" align="center">6</td>
</tr>
</table>
You can change number of rows and columns and see how background image expand or shrink, here, in fiddle example
In this example, background image is 300x300 px.