Structure of MySQL query is following:
UPDATE `table_name` SET `field_name` = replace(field_name, 'pixhost.org', 'pixhost.to');

Wordpress query:
UPDATE `wp_posts` SET `post_content` = replace(post_content, 'pixhost.org', 'pixhost.to');

phpBB3 query:
UPDATE `phpbb_posts` SET `post_text` = replace(post_text, 'pixhost.org', 'pixhost.to');