adding a logo

The following information is required to get an answer:
Get this information from the PHP diagnostic (in the configuration wizard).
Gallery URL (http://kang0o.net/gallery):
Gallery version: 1.5.7
Apache version:
PHP version (4.4.7):
Graphics Toolkit:NetPBM
Operating system:Linux
Web browser/version (if applicable):

I'm sure this must of been asked a zillion times, but I'll be jiggered if I can find a thread to help me.

I installed Gallery1 today and have opted for the 'black' skin (looks very nice). Can someone tell me the easiest way to add my site logo to the page. I want it top left above the title bar.
I have always built my own pages before and very simple html dreamweaver ones at that. I have had some experience of these self installing php scripts, but it has always been a case of looking through the theme package I've chosen, finding the logo image, making my own, naming the the same and then just overwriting the old file. But the black skin doesn't have a logo so that won't work.
I really don't have a lot of knowledge of css, but I seem to remember on a phpwebsite install me and a friend did we used the image tags to link to a file from the css. I really can't remember how I did it though.
I don't want to start will nilly overwriting server files even if I back it up, is all to easy to get muddled and break something.
I know this probably all sounds a bit daft, but I could use some help.

I need to change in the banner logo and url address.
I'm using Gallery version 1.3
I tried to edit in themes/matrix/theme.tpl but it doesn't work.
Here is my code.
===================================================================
{if $theme.useFullScreen}
{include file="gallery:`$theme.moduleTemplate`" l10Domain=$theme.moduleL10Domain}
{elseif $theme.pageType == 'progressbar'}
<div id="gsHeader"><a href="http://localhost/mydrupal5">
<img src="images/galleryLogo_sm.gif" width="107" height="48" alt=""/></a>
</div>
{g->theme include="progressbar.tpl"}
{else}
<div id="gsHeader">
<a href="http://localhost/mydrupal5"><img src="images/galleryLogo_sm.gif"
width="107" height="48" alt=""/></a>
</div>

<div id="gsNavBar" class="gcBorder1">
<div class="gbSystemLinks">
{g->block type="core.SystemLinks"
order="core.SiteAdmin core.YourAccount core.Login core.Logout"
othersAt=4}
</div>

<div class="gbBreadCrumb">
{g->block type="core.BreadCrumb"}
</div>
</div>

{* Include the appropriate content type for the page we want to draw. *}
{if $theme.pageType == 'album'}
{g->theme include="album.tpl"}
{elseif $theme.pageType == 'photo'}
{g->theme include="photo.tpl"}
{elseif $theme.pageType == 'admin'}
{g->theme include="admin.tpl"}
{elseif $theme.pageType == 'module'}
{g->theme include="module.tpl"}
{/if}