Well from what I know Under international law,freedom of speech
is required to comport with a strict three part test:
1. they must be provided by law;
2. pursue an aim recognized as legitimate;
3. they must be necessary (i.e., proportionate) for the accomplishment of that aim
Amongst the aims considered legitimate are protection of the rights and reputations of others (prevention of defamation), and the protection of national security and public order, health and morals.
What does defamation mean?
Defamation can be categorised as Libel or slander which means false statement that negatively affects someone's reputation in this case the cartoons affect the prophet's reputation and to its ummah.
According to the American and English Encyclopedia of Law, a libel is a malicious defamation expressed either by writing or printing or by signs, pictures, effigies or the like; tending to blacken the memory of one who is dead, or to impeach the honesty, integrity, virtue or reputation, or to publish the natural or alleged defects of one who is alive, thereby exposing him to public hatred, contempt, ridicule or obloquy; or to cause him to be avoided or shunned or to injure him in his office, business or occupation.
I hope this explanation is clear enough of what the limits of freedom of speech are.
