I am developing a simple photo editing app. I would like a journal feature for the user to view each of the photos they've edited. Some power user might have hundreds (or potentially thousands) of photos.
I am using Core Data to save photos. My concern is that hundreds of 640x640 photos might make the documents directory filesize rather large (side question: My assumption is my core data store is saved to my apps Documents directory. Is that true?).
Is it a problem to store so many photos locally within the app or am I being overly cautious? Is there a filesize limit on how much I can store?
There is no limit for storing data in the documents directory (supposing you're using documents for data and core data for paths). There was a discussion on that in this question. Other than that, you'll be fine, working with data from documents directory is rather fast by default.
The thing is - will your users be happy with your app getting bigger and bigger? Can you perhaps give them the option to store the pictures on the server and let them set the cache size?
OR - you could go for storing it in the photo library if you get stuck with it...