Forget the idea of columns please.
The number of posts you set to display here > Settings > Reading determines how many posts will display on the front page of the blog. It's by setting featured images at least 350 by 248 (width, height) as sticky posts that determines how many posts display side-by-side. Two posts each with featured images results in what you refer to as two columns.
Suburbia description notes http://theme.wordpress.com/themes/suburbia/
live demo site http://suburbiademo.wordpress.com/
On the main page, Suburbia doesn't display regularly inserted images. They only display when a post is viewed on its own page - not on the front page. Only two posts with featured images in them display there side by side.
To have front page image display you need to set a featured image to each post and make each post a sticky post. The size of the featured image does matter. The featured image works best with images at least 350 by 248 (width, height).
Setting a Featured Image > http://en.support.wordpress.com/featured-images/#setting-a-featured-image
Stcky posts > http://en.support.wordpress.com/posts/post-visibility/#sticky-posts
If you don’t see the Featured Image module on your Add New or Edit Post page, please make sure you’ve selected Featured Image in your Screen Options. http://en.support.wordpress.com/screen-options