Is prayer permissible in a mosque affiliated with a Turkish cultural association that glorifies Mustafa Kemal Atatürk?

The prayer behind the aforementioned Imam is valid as long as he is a Muslim. Even if his transgression (fisq) were proven, prayer behind him would still be valid, though disliked. The mere hanging of a picture of Atatürk is not proof of his glorification; he may be unable to remove it or ignorant of his true nature. The default assumption for a Muslim is blamelessness. They should be advised gently and wisely to remove the picture.