It's not mentioned in Islam's holy book, the Koran, but the religion's ban on depicting the Prophet Mohammed - even favourably - has run firm through the centuries.
Religious traditions built over the years have prohibited such depictions out of respect for Mohammed and to discourage idolatry, according to Muslim scholars and clerics. The ban is further rooted in a wider prohibition against images or statues of human beings.
There have been exceptions. A rich tradition of depicting Mohammed emerged in miniatures and illustrations for manuscripts from around 1200 to 1700.
The art is mainly from Turkey and Iran, where pictorial traditions were stronger than in the Arab world. The paintings often show traditional stories from Mohammed's life, such as his journey to heaven, though in some the prophet's face is obscured by a veil or a plume of flame.
Shi'ites also differ from Sunnis by depicting Mohammed's son-in-law Ali, revered by Shi'ites who see him as the prophet's rightful successor. His image - and those of his sons Hassan and Hussein - are plentiful among Shi'ites, adorning posters, banners, jewellery and even keychains.
For Sunnis, the ban on depictions extends beyond the prophet to his close companions and wives.
"The Prophet Mohammed enjoys sublime and supreme status among Muslims and it is impossible to let a normal person depict or act the role of the prophet," said Iraqi Shi'ite cleric Fadhil al-Saadi.
"There is no confirmed information about the shape or the features of the Prophet ... so nobody should come up with a painting or an image of him. That would represent an insult to the status of the prophet."
With no explicit text against depictions - or against images of humans in general - the prohibition comes from deduction by Muslim scholars and interpreters over the centuries from the collections of Hadith, or sayings and actions of Mohammed.
The prohibition against depicting humans and other living beings, which emerged from scholars as early as the ninth century, came from reported sayings of Mohammed, in some of which he refused to enter a room with such depictions or challenged their creators to breathe life into them.
The presumption was that such art would suggest man can emulate God's powers of creation - and there were worries that statues in particular could encourage idolatry.
Islamic tradition is full of written descriptions of Mohammed and his qualities - describing him as the ideal human being. But clerics have generally agreed that trying to depict that ideal is forbidden. That puts satirical - and obscene - depictions like those in the French magazine Charlie Hebdo far beyond the pale.
While no one knows Mohammed's true appearance, followers of the relatively modern, ultraconservative Salafi movement in Islam seek to emulate him as closely as possible - including in what they believe to be his physical features and dress. Hardcore Salafis wear a beard without a moustache, let their hair grow long, line their eyes with kohl or wear robes stopping around mid-shin, contending that was the prophet's manner.
The ban also extends to his wives, daughters, sons-in-law, the first caliphs who succeeded him and his closest companions. In fact, Egypt's al-Azhar mosque, the Sunni world's foremost seat of religious learning, has complained when Mohammed, Messenger of God, an epic 1970s Hollywood production, depicted the prophet's camel.
There is a thriving production of religious TV series in the Arab world depicting the times of the prophet. But Mohammed and his companions are never themselves shown. At times, a white light stands in for Mohammed in the films or in movie posters - and when they are meant to be addressing Mohammed, the actors usually speak into the camera.
