Also, is there a way to make sure the image is in the middle?
YES – Create a stylesheet! You still have main.css with nothing in it. Did you read the article I linked to above?
<img id="logo" means there should be
#logo in your stylesheet with rules for that logo ID (see Inger's example above). Until you do that, your logo is staying right where it is.
You're going to need to set some styles for your menus as well, so you better get going on learning how to write your CSS.
That background image is HUGE. Just because it works fine for you doesn't mean everyone else will see it the same way.
http://www.webresizer.com/resizer/
(Also, you still have the period in your file name (src="images/i.d_logo.jpg"), that needs to go.)