Hi Steve,
Remember to assign your domain to your licensed extensions to obtain support for the domain.

As for your issue, the way the importer works is that it searches for the media:thumbnail tag in the provided url of the rss feed.
If you view the page source of the rss feed url: view-source:http://thedeathjungle.blogspot.fr/feeds/posts/default/-/Blood%20bowl/?alt=rss, you can see that the thumbnail image provided by the site has a low resolution(http://take.ms/RjtLZ).
Because of this, I would suggest you to disable the 'import image as post cover' setting for this site.