Oulipo theme header image not working

I have a nice custom header on my wordpress.com Oulipo theme:

http://www.cityparkstudio.wordpress.com

but it won’t work on my self-hosted site, which I’m trying to get up and running:

http://www.cityparkstudio.com/blog

it seems to be giving a small 120×160 pixel block instead of a full width header..

help appreciated