Isn't Freedom of Religion written somewhere in the US constitution?

Not exactly.

Does that not mean that as a religion, you get to decide what is acceptable and what is not, within the law of the land! Or does Freedom of Religion really mean you can be whatever kind of Christian you want!

There is nothing exactly called "freedom of religion".   We have "separation of church and state", otherwise known as the "Establishment Clause", i.e., "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof".

What this means is that the government cannot favour one religion over another or tell you what to believe or how to practice.

Making a statue of Muhammad, or, for that matter, a cartoon of him, does not violate that clause.