[Wordpress] Pre-set default image sizes (programatically), thus sizes become fixed & unchangable.

wp-img-size-preset.php

<?php

update_option( 'thumbnail_size_h', 150 );
update_option( 'thumbnail_size_w', 150 );
update_option( 'medium_size_h', 500 );
update_option( 'medium_size_w', 500 );
update_option( 'large_size_h', 1000 );
update_option( 'large_size_w', 1000 );