Question

When should I use each?

Answer 1

NSImage is an abstract data type that can represent many different types of images, as well as multiple representations of an image. It is often useful when the actual type of image is not important for what you’re trying to do. It is also the only image class that AppKit will accept in its APIs (NSImageView and so forth).

CGImage can only represent bitmaps. If you need to get down and dirty with the actual bitmap data, CGImage is an appropriate type to use. The operations in CoreGraphics, such as blend modes and masking, require CGImageRefs. CGImageRefs can be used to create NSBitmapImageRefs, which can then added to an NSImage.

I think the documentation describes a CIImage best:

Although a CIImage object has image data associated with it, it is not an image. You can think of a CIImage object as an image “recipe.” A CIImage object has all the information necessary to produce an image, but Core Image doesn’t actually render an image until it is told to do so. This “lazy evaluation” method allows Core Image to operate as efficiently as possible.

CIImages are the type required to use the various GPU-optimized Core Image filters that come with Mac OS X, but, like CGImageRefs, they can also be converted to NSBitmapImageReps.