Which isn't exactly a terrorist flag.
Historically speaking, Islam doesn't necessarily have a single image to represent it. You've the black/white with Shahada. You've got green banners, as well as other colors like white and red. You also have the crescent(which is probably the most universally accepted of being representative of Islam, and was brought about by the Ottoman Empire). The crescent star is also used by some, but the crescent itself is commonly accepted.