It is a mini project I got to work on GCD.
So the project loads a tableview of list of Guests type. In the cell, on main thread, label loads the name of the guests from firebase , and the imageView loads a placeholder image temperily. and on the separated thread the photos of the guests(a cat picture in this sample) being loaded and replace the placeholder image once it is done downloading from Firebase storeage.
For more information, you can refer to my blog http://lingios.blogspot.com/2016/08/grand-central-dispatch.html