Returns a Boolean value indicating whether the receiver retains its source image data.
YES if the image retains its source data; otherwise, NO. The default value is NO with some exceptions, which are covered in the discussion.
For image objects initialized using either the initByReferencingFile: or initByReferencingURL: method, this value is YES by default. The reason is that for these methods, data retention simply involves retaining the filename or URL.
Data retention increases the memory used by the NSImage object and its image representations.