First, always store your images in a usable format such as PNG or JPEG instead of NSData. This will save you a lot of headaches.
Second, the rule for storing binary data is:
- < 100kb store in the same table as the relevant data
- < 1mb store in a separate table attached via a relationship to avoid loading unnecessarily
-
1mb store on disk and reference it inside of Core Data
Update
The storage inside of Core Data should be binary and you can write accessor methods for it. Take a look at this answer: Core data images from desktop to iphone
Update
The example code I linked to describes how to create accessors in your NSManagedObject subclass that will convert the image back and forth between a UIImage and binary data.