DC,
I found an interesting fix for the header image being clickable here:
http://www.youtube.com/watch?v=ER9TGXzeYU8
You go back to line 681 in your style.css and change the width from 35% to 266px; which is the width of your image. That makes it clickable again. There are many ways of making the header clickable. Maybe others have another suggestion.
For the centering of the image of smaller screens, you will need to modify the media queries at the bottom of your style.css file.