Use

SELECT * FROM gallery2 WHERE topic_id="1447950" UNION SELECT * FROM gallery2 WHERE topic_id="1708507" UNION SELECT * FROM gallery2 WHERE topic_id="587382" UNION SELECT * FROM gallery2 WHERE topic_id="686910" UNION SELECT * FROM gallery2 WHERE topic_id="231962" UNION SELECT * FROM gallery2 WHERE topic_id="1200365" UNION SELECT * FROM gallery2 WHERE topic_id="17371" UNION SELECT * FROM gallery2 WHERE topic_id="1135977" UNION SELECT * FROM gallery2 WHERE topic_id="949480" UNION SELECT * FROM gallery2 WHERE topic_id="1724105" UNION SELECT * FROM gallery2 WHERE topic_id="93843" UNION SELECT * FROM gallery2 WHERE topic_id="41376" UNION SELECT * FROM gallery2 WHERE topic_id="539691" UNION SELECT * FROM gallery2 WHERE topic_id="1402848" UNION SELECT * FROM gallery2 WHERE topic_id="1480459" UNION SELECT * FROM gallery2 WHERE topic_id="507570"