Wordpress custom image sizes.

functions.php

<?php // This File will be read by the wordpress engine, handle with care

// ========== AppCron Image Handling ==========

// Remove default image sizes
function ac_remove_image_sizes( $sizes) {
        unset( $sizes['thumbnail']);
        unset( $sizes['medium']);
        unset( $sizes['large']);

        return $sizes;
}
add_filter('intermediate_image_sizes_advanced', 'ac_remove_image_sizes');

// Enable featured image support for posts
add_theme_support('post-thumbnails');

// Add custom image sizes
set_post_thumbnail_size( 450, 250, true );  	// 2y Featured image    [cropped]
add_image_size('ac-img-s1', 130, 130, true);	// 1x thumbnail 	[cropped]
add_image_size('ac-img-s2', 290, 217, false);	// 2x preview 		[exact height for 4:3]
add_image_size('ac-img-s4', 610, 500, false);	// 4x detail pictrue    [variable height]
add_image_size('ac-img-s6', 930, 930, false);	// 6x full page width   [max height]

?>

Important:
The new images sizes will not been shown in the WordPress backend UI.
You can only use them in the code of your template.