The image depicts a black banner with the shahada (Islamic testimony of faith holding that there is no god but Allah and that Muhammad is his messenger) and an AK47 underneath. The image might be construed as a jihadist retort to the flag of Saudi Arabia, which bears a classic Arabian sword under the text of the shahada.
According to tradition hadith (prophetic tradition or report), the black flag was the battle flag of the Prophet Muhammad and it was carried into battle by many of his companions. The image of the black flag has been used as a symbol of religious revolt and engagement in battle (i.e., jihad). In the contemporary Islamist movement, the black flag is used to evoke notions of jihad and of reestablishing the Islamic Caliphate.