Not really sure about posterous, but yes, you can use windows live writer. I am still using windows office 2007, but I guess the 2010 version already has something to solve your problem too, I'm just not really sure about that as well though.
I also asked myself about this cause I'm not a pro on html; and yes, Windows Live Writer was the answer to your dilemma. It is a free application from Microsoft. You can create beautiful blog posts, and see what they'll look like online before you publish them to your blog. Plus, you can publish your posts to any of your favorite blog service providers.