How to Place a Graphic Logo at the Top of Your Blog
See how my logo sits at the top of this blog (rather than the blog title, written as text)? That took a little work. Good news is, it’s far from rocket science.
Here’s how to place a graphic logo at the top of your Wordpress blog:
1) Upload your logo, as a .jpg. I suggest giving your logo a simple, descriptive filename, such as “Company_Name_Logo.jpg”.
2) Copy the full URL of your logo to your clipboard (including the “http://”).
3) Open your “header.php” file (which is located in your Wordpress Admin Panel / Appearance / Editor / Header).
4) Scroll down to near the bottom and find the "<h1 class=", and after the “=”, insert the following:
<a href=”<?php bloginfo(’url’); ?>”><img src=”http://INSERT_COPIED_LINK_HERE” alt=”" /></a>
5) Save the modified PHP file.
DONE!
Note: To update your logo, simply give the new logo the same filename as your old logo, and upload it into the same folder as your old logo, over-writing it. [Which is why I suggested giving your logo a simple, descriptive name.]
http://www.adobe-masters.com/tutorials/how-to-add-logo-to-wordpress-site-and-show-code-in-worpress/

Thanks but I think you’re referring to header.php (css files don’t contain html markup)
cheers for the help tho, got it done: LargerSystems.com.au
Gerry