Mainstream Islamic practice holds that God, the Prophet Muhammad, and by extension the Prophet's closest companions should not be rendered as pictures, a restriction rooted in concern that a visual image invites the kind of veneration properly owed to God alone. That leaves a real design problem for anyone building a space meant to honor those same figures: how do you give a worshipper somewhere to look?
The Sultan Ahmed Mosque answers with writing rather than pictures. Its roundels hang exactly where a Christian church of the same era might place a painted image, high on the piers that visually carry the weight of the whole dome, but instead of a face they present a name, rendered so large and in such deliberately monumental script that the calligraphy itself becomes the object worth looking at. Allah, Muhammad, the four Rightly Guided Caliphs and the Prophet's two grandsons are each given their own roundel, arranged so that the structural piers holding up the building and the sacred names holding up the faith read as one and the same image.
The same restriction shows up in other media too: The Message, Moustapha Akkad's 1976 film on the origins of Islam, never shows Muhammad on screen at all, letting his uncle Hamza and his companions carry the story instead. Architecture writes his name in gold; film simply declines to point the camera at him. Two very different technologies arriving, independently, at the same restraint.