By Amy Sawitta Lefevre BANGKOK (Reuters) - Thailand has asked Interpol for help in tracking down the man they believe planted a bomb in Bangkok that killed 20 people, sending the international police organization an image of the suspected bomber. "We sent a request for assistance," deputy national police spokesman Kissana Phathancharoen told Reuters. There has been no claim of responsibility and police have not determined a motive for the worst ever bomb attack in Thailand.