Hi, concepta.
That’s because the main content area – i.e. the area under the header and above the footer – has 40px of inbuilt padding to keep the content away from the hard edges.
If you want some content to be flush with the hard left edge of the main content area then you can add to your HTML content a div with a negative left margin to do that. For example…
<div style="margin-left: -40px;">
<p style="text-align: center;">
<a href="http://headstarthairsalontrim.com/wp-content/uploads/2014/04/GIFT-VOUCHER-GRAPHIC.jpg">
<img class="size-full wp-image-128 aligncenter" alt="GIFT VOUCHER GRAPHIC" src="http://headstarthairsalontrim.com/wp-content/uploads/2014/04/GIFT-VOUCHER-GRAPHIC.jpg" width="960" height="175" />
</a>
</p>
</div>