+imageNamed 會cache memory直到low memory才會釋放

• The -initWithContentsOfFile: creates a new image without caching, it's an ordinary initialization method.
• The +imageNamed: method uses cache. Here's a documentation from UIImage Reference:
  

  This method looks in the system caches for an image object with the specified name and returns that object if it exists. If a matching image object is not already in the cache, this method loads the image data from the specified file, caches it, and then returns the resulting object.
  

  UIImage will retain loaded image, keeping it alive until low memory condition will cause the cache to be purged.