Hi,
Afterburner is a theme optimized for speed and performance.Faster Than WordPress Default Theme: the theme focuses on performance from techniques such as sprites – using a single image for all the images on the site which lowers HTTP requests – to a streamlined, optimised CSS and PHP structure and many other methods to make Afterburner lightning fast.
That's why it's all in one image. You can change all dimensions of logo in css. And here's a tutorial on changing logo :
demo.rockettheme.com/wordpress/wp_afterb...orials/logo-editing/
Jakub