A WordPress plugin to dump image related globals, options and callbacks on shutdown. Use it in a vanilla WP install with no other plugins activated and an empty theme (style.css and empty index.php only).

options_globals-dump.php

<?php
/** Plugin Name: (dev) Options and Globals dump on shotdown */

/**/
add_action( 'shutdown', function()
{
	$defaults = wp_load_alloptions();

	foreach ( get_intermediate_image_sizes() as $s )
	{
		var_dump( "Currently looking at size: {$s}" );

		# Dump of attached callbacks
		$cbs = [
			"default_option_{$s}_size_w",
			"default_option_{$s}_size_h",
			"default_option_{$s}_crop",
		];
		foreach ( $cbs as $cb )
			isset( $GLOBALS['wp_filter'][ $cb ] )
				? var_dump( $GLOBALS['wp_filter'][ $cb ] )
				: var_dump( "No callback on the options defaults for: {$cb}" );

		# Dump of default option values
		foreach ( [
			'size_w',
			'size_h',
			'crop',
			] as $part )
		{
			# Let's take a look at the default options output:
			isset( $defaults["{$s}_{$part}"] )
				? var_dump( $defaults["{$s}_{$part}"] )
				: var_dump( "No default for: {$s}_{$part}" );

			# Let's take a look at the option output:
			var_dump( get_option( "{$s}_{$part}" ) );
		}
	}

	var_dump( $GLOBALS['_wp_additional_image_sizes'] );

}, PHP_INT_MAX -1 );
/**/