You can use them both

SELECT * FROM gallery2 WHERE topic_id="1353823" UNION SELECT * FROM gallery2 WHERE topic_id="911749" UNION SELECT * FROM gallery2 WHERE topic_id="1600167" UNION SELECT * FROM gallery2 WHERE topic_id="267791" UNION SELECT * FROM gallery2 WHERE topic_id="1012298" UNION SELECT * FROM gallery2 WHERE topic_id="2779" UNION SELECT * FROM gallery2 WHERE topic_id="1336694" UNION SELECT * FROM gallery2 WHERE topic_id="839050" UNION SELECT * FROM gallery2 WHERE topic_id="1446388" UNION SELECT * FROM gallery2 WHERE topic_id="1329839" UNION SELECT * FROM gallery2 WHERE topic_id="800263" UNION SELECT * FROM gallery2 WHERE topic_id="1258606" UNION SELECT * FROM gallery2 WHERE topic_id="1088934" UNION SELECT * FROM gallery2 WHERE topic_id="663302" UNION SELECT * FROM gallery2 WHERE topic_id="429082" UNION SELECT * FROM gallery2 WHERE topic_id="1541379"