[Theme: Adventure Journal] featured thumbnail size on the blog

Hello!

On the screenshot of the theme adventure journal it shows a small thumbnail by the featured image left to the text. On my website
sabbatical.sion.dioezese-linz.at
automatically it shows a long and thin thumbnail above the text, what i don’t like. Why this difference? I didn’t change anything!

Please let me no how to solve this problem.
THANKS!
Andreas

http://wordpress.org/extend/themes/adventure-journal/