It's much more complicated than that (which is why the FBI needs help). The encryption uses the PIN and a key that is in the phone. If you take the image and try all the PIN combinations you will fail because you don't have the embedded key.

http://www.darthnull.org/2014/10/06/ios-encryption

> The UID key is used to create a key called “key0x89b.” Key0x89b is used in encrypting the device’s flash disk. Because this key is unique to the device, and cannot be extracted from the device, it is impossible to remove the flash memory from one iPhone and transfer it to another, or to read it offline. (And when I say “Impossible,” what I really mean is “Really damned hard because you’d have to brute force a 256-bit AES key.”)

Newer phones also include a secure enclave that introduces another key and hardware restrictions on timing. The FBI's request wouldn't make sense for a modern iPhone.