In your case it should be, I think, url('wordpress/wp-images/myheaderimage.jpg') . Or use the full path: url('http://petite-anglaise.com/wordpress/wp-images/....jpg').
If I remember right, Root uses #masthead for the blog header div.
As for importing first or later, it doesn't really matter. Once you've defined your basic stylistic choices, you could import in order to see the effect of your layout on real life posts.