Replacing theme type header w/ image in CSS?

Hello all,
I’m new to CSS & not familiar w/ PHP so any advice would be greatly appreciated.
I’d like to replace my Sharpfolio theme type based header eventually with a image header – my header php posted below is where I assume I’d place the new code(?)

<div id=”wrapper”>

<div id=”header” class=”clear”>
<h1>“><?php bloginfo(‘name’); ?></h1>
<ul id=”navigation”>

• “>< previous

<?php wp_list_pages(‘&title_li=’); ?>

• “>RSS Feed

<h2><?php bloginfo(‘description’); ?></h2>
</div>

Also where do I place the size parameters?
– As an example 800 x 260 pixels

thanks so much in advance – Johnny