A very good thing of the "je suis charlie” message is that
it can be endorsed and adopted by moderate muslims.
Terrorism can't be combated by throwing bombs at targets, nor by elevating the discussion level to extreme confrontation; those actions just escalate the spiral of violence. Historical experiences, like the recent disempowerment of ETA in Spain, show that terrorism is defeated by insulating the radicals from the community whose interests they purport to represent.
That so many muslims in Europe have condemned the attack and shared the phrase is the best answer we could hope for. If only the same response had been made throughout the Islamic world as well, these undesirables would known that they can't claim any moral support, and soon they would find themselves without economic support as well.
That simply couldn't happen if our reaction was to place an emphasis on the images that are offensive to islamists. The ideals we value are that we're free to express and discuss any idea (even when it's despicable) without facing prosecution from authority, not that we can say things intended to hurt a different religion with impunity; the second is merely a by-product of the first, and it's OK that people may hold a strong disagreement with those expressions, as long as they are pursued in a peaceful manner.