You create new instances of ImageIO.CGImageDestination by calling one of the factory methods.   There are three versions of it:

• Storing the generated image into a file pointed to by an NSUrl.
• Storing the generated image into an Foundation.NSMutableData
• Storing the generated image into a CoreGraphics.CGDataConsumer

In the classic API, those methods were called FromUrl, FromData and Create respectively.   But this naming was incorrect, as it did not really create an image destination from a url, or a data.  In the Unified API, they have all been turned into CGImageDestination.Create methods.

Once you have created the image, you can call CGImageDestination.AddImage or CGImageDestination.AddImageAndMetadata to add one or more images.   

To write out the image, you must call the CGImageDestination.Close method.   

// The following example shows how to store an image into
// an in-memory PNG file stored inside an NSData.

NSData ImageToPng (CGImage image)
{
   var storage = new NSMutableData ()
   var dest = CGImageDestination.Create (storage, MobileCoreServices.UTType.PNG, imageCount: 1);
   dest.AddImage (image);
   dest.Close (); 
   return storage;
}