The black banner with the takbir phrase (the words “Allahu Akbar” or “God is great”) is a recognizable part of the Jaysh al-Mujahidin’s logo. The caption reads (from the top): “tahunu al-hayat wa-kull[un] yahunu wa-lakin islamuna la yahunu” (“life becomes contemptible and everything weakens, but our religion [lit. Islam] does not weaken”). This is a line from a famous jiahdist nashid (song). Nashid are frequently used in jihadist videos or are released directly onto the forums. This particular image is the signature for a forum user.
According to prophetic tradition (hadith), the black flag was the battle flag of the Prophet Muhammad and it was carried into battle by many of his companions. In the contemporary Islamist movement, the black flag with the shahada (Islamic testimony of faith holding that there is no god but Allah and that Muhammad is his messenger) or takbir (the words “Allahu Akbar” or “God is great”) is used to evoke notions of jihad and of reestablishing the Islamic Caliphate.