Actually you don't need to change the name or type of your file. If you find the following line in includes/languages/english/YOUR_TEMPLATE_NAME/header.php
define('HEADER_LOGO_IMAGE', 'logo.gif');
You can set your existing filename to be picked up as the logo file. You would then need to put your file in includes/templates/YOUR_TEMPLATE_NAME/images. (admin/images/logo.gif is where the logo that is displayed when you log in to admin is kept.)

The answers to your second post are immediately above it in the following lines
define('HEADER_SALES_TEXT', 'TagLine Here');
define('HEADER_LOGO_WIDTH', '192px');
define('HEADER_LOGO_HEIGHT', '64px');