If your application has server-side avia, this problem can be solved on the server side by creating a lower-quality preview thumbnail that will be sent to your client application, you still need to manually track which files you have. In this case SDWebImage may be useful (url: http://cocoadocs.org/docsets/SDWebImage/3.7.5/ ).
The problem is that there is no way to preview the image without actually loading the data itself, so why my bet will be on the server side of this solution.