[Victor D]

skin1/head.tpl
change width and height according to your image.
<img src="{$ImagesDir}/xlogo.gif" width="244" height="67" alt="" />

as you wish this logo to have width similar with the total page width
you need to change the table structure of the first table in this file

<table cellpadding="0" cellspacing="0" width="100%">
<tr> 
    <td class="HeadLogo"><a href="{$http_location}/"><img src="{$ImagesDir}/xlogo.gif" width="244" height="67" alt="" /></a></td>
    <td class="HeadRightBox">
{if $usertype eq "C"}
{include file="customer/top_menu.tpl"}
{/if}
    </td>
</tr>
</table>

to

<table cellpadding="0" cellspacing="0" width="100%">
<tr> 
    <td width="100%"><a href="{$http_location}/"><img src="{$ImagesDir}/xlogo.gif" width="244" height="67" alt="" /></a></td>
</tr>
{if $usertype eq "C"}
<tr>
    <td class="HeadRightBox">
{include file="customer/top_menu.tpl"}
    </td>
</tr>
{/if}
</table>