In Xcode, when I\'m trying to add more than 5 images to my library, it gives me the following error:
Error Domain=ALAssetsLibraryErrorDomain
I'd recommend using an NSOperationQueue and play with the value of maxConcurrentOperationCount. This way you can control the number of simultaneous writes to the library, and not overwhelm it.
If you use threads, or even GCD, you'd need to implement this logic yourself. More code --> more chance of introducing a bug.