WordPress | Create checkboxes on Settings > Media which crop medium and large image sizes.

functions_crop-images.php

// Add crop options for medium and large images to dashboard > settings > media.
function crop_settings_api_init() {
	// Add the section to media settings
	add_settings_section(
		'crop_settings_section',
		'Crop images',
		'crop_settings_callback_function',
		'media'
	);
	// Add the fields to the new section
	add_settings_field(
		'medium_crop',
		'Medium size crop',
		'crop_medium_callback_function',
		'media',
		'crop_settings_section'
	);
	add_settings_field(
		'large_crop',
		'Large size crop',
		'crop_large_callback_function',
		'media',
		'crop_settings_section'
	);
	register_setting( 'media', 'medium_crop' );
	register_setting( 'media', 'large_crop' );
} // crop_settings_api_init()

add_action( 'admin_init', 'crop_settings_api_init', 1 );

// Settings section callback function
function crop_settings_callback_function() {
    echo '<p>Choose whether to crop the medium and large size images</p>';
}
// Callback function for our medium crop setting
function crop_medium_callback_function() {
	echo '<input name="medium_crop" type="checkbox" id="medium_crop" value="1"';
    $mediumcrop = get_option( "medium_crop");
    if ( $mediumcrop == 1 ) {
        echo ' checked';
	} 
	echo '/>';
	echo '<label for="medium_crop">Crop medium to exact dimensions</label>';
}
// Callback function for our large crop setting
function crop_large_callback_function() {
	echo '<input name="large_crop" type="checkbox" id="large_crop" value="1"';
    $largecrop = get_option( "large_crop");
    if ( $largecrop == 1 ) {
        echo ' checked';
	} 
	echo '/>';
	echo '<label for="large_crop">Crop large to exact dimensions</label>';
}

More than a thumbnail crop

For many years it has bugged me that, for each image size within media, there's not a crop option.

So I have cobbled together this, which essentially adds a new section to the existing Settings > Media page called Crop images.

It's that simple, just add the above code to your themes existing functions.php file and you'll see the new section appear.

Developers, if you're after a less bulky solution, know you'll always want the crop to happen and have no interest in adding options to settings. You can simply add this to your functions file:

if ( get_option( "medium_crop") !== false ) {
    update_option("medium_crop", "1");
} else {
    add_option("medium_crop", "1");
}

This works, thanks! This should be in Wordpress core. I think a rogue plugin switched medium and large images to crop on my install, and this was the only simple way to reset that.

Really glad it helped you out @earthbound19, thanks for commenting!

This is very cool, if I can get it to work. I see the options in the Media Settings and it says it's saving, but my medium images aren't showing the new size. I've tried calling 'medium' and 'medium_crop' but the images are still the old medium size (300px x 300px) after I've updated the size to 420px x 420px and cropped. Do I need to reupload the images or something? Thanks, man!

@morgyface, I just found your medium-crop-added-to-settings-media thread on wordpress.org. I could kiss you. In fact I'm going to if I ever see you, so watch out.

@televators if you haven't already, install a regenerate thumbnail plugin (like this one: https://wordpress.org/plugins/regenerate-thumbnails/) and run it to create your new crops

Sorry @televators, I should've explained you need to re-process your image library once you've got this in place. You can either delete and re-add the images in question, if there's only a couple, or you can use something like the regenerate thumbnail plugin the wondrous @zaccety has mentioned. That'll rattle through the whole media library cropping as it goes.