What you need to do is the following:

Save the logo picture you want to the following directory:

includes/templates/the template you are using/images

Name it something new that is not already in there, this way you wont overwrite any existing images.

Then, go to:

includes/languages/english/the template that your using/header.php

define('HEADER_LOGO_WIDTH', '700px');
define('HEADER_LOGO_HEIGHT', '140px');
define('HEADER_LOGO_IMAGE', 'Image5.png');

Add the image width/height and filename as saved previously...

Steve