Please read the theme description here > http://theme.wordpress.com/themes/origin/
Origin is a light, elegant theme with a minimalist look and feel, perfect for a blog or journal. Your photos are the focus thanks to featured images and stylized Sticky posts, and the responsive design makes it a great fit for mobile devices and small screen sizes.
If your post has a Featured Image set, it will display as a post thumbnail on the blog index. Post thumbnails are 154 pixels wide, unless you make the post Sticky, in which case they’re the full column width of 640 pixels.