WordPress: Filter default image sizes on read to set custom sizes in a theme. Based on approach by Tammy Hart: http://10up.com/blog/2012/12/enforcing-wordpress-image-sizes-within-your-theme/

gistfile1.php

<?php
add_filter( 'pre_option_thumbnail_crop', 'themename_default_image_options' );
add_filter( 'pre_option_thumbnail_size_h', 'themename_default_image_options' );
add_filter( 'pre_option_thumbnail_size_w', 'themename_default_image_options' );
add_filter( 'pre_option_medium_size_h', 'themename_default_image_options' );
add_filter( 'pre_option_medium_size_w', 'themename_default_image_options' );
add_filter( 'pre_option_large_size_h', 'themename_default_image_options' );
add_filter( 'pre_option_large_size_w', 'themename_default_image_options' );

function themename_default_image_options( $value ) {
	$option = str_replace( 'pre_option_', '', current_filter() );
	
	switch ( $option ) {
		case 'thumbnail_size_h' :
		case 'thumbnail_size_w' :
			$value = 180;
			break;
		case 'thumbnail_crop' :
			$value = true;
			break;
		case 'medium_size_h' :
		case 'medium_size_w' :
			$value = 390;
			break;
		case 'large_size_h' :
		case 'large_size_w' :
			$value = 660;
			break;
	}
	
	return $value;
}

For thumbnail_crop it should return a number: 0 or 1