WP, php, CSS, functions.php: $content_width in functions.php required

new_gist_file

if ( ! isset( $content_width ) )
	$content_width = 600;

It's also recommended to add the following CSS to your theme:

.size-auto, 
.size-full,
.size-large,
.size-medium,
.size-thumbnail {
  max-width: 100%;
	height: auto;
}