Fullscreen thene

Hi there,
I’m starting a new blog in wordpress.org. I already have a blog at wordpress.com and I would like to move some posts from on blog to the other.
My problem is that export/import doesn’t seem to generate the thumbnails that my new theme (Fullscreen) needs to fill the main page.
Is there any way to do that during import or afterwards with a plugin ?
Help is higlly appreciated!
Thanks,
Pedro