Is your client firm on that? No one really does that “enter site” approach anymore. Just a suggestion as it’s a bad UX approach. Have you tried to compress your background image so it doesn’t take so long to load?
I’m not sure about scaling the image. I would guess using Media Queries and use a transition to make it look smooth when the viewport changes.
As for vertically and horizontally centering, @jamy_za created the code for me. He basically used absolute positioning, setting the top and left at 50%.
I’m strictly a designer so I wouldn’t want to waste your time trying something that wouldn’t work off the bat. I’m sure someone will be around to help you soon. Sorry!