As I say in the title of the question, I'm developing a universal app. At this point, the app has reached what I think it is a considerable size (some more than 30 MB), because I have quite a lot of images and icons, and for both versions iPhone and iPad. In fact, I have two versions of each image and icon (iPhone size / iPad size), and, in turn, I have also two sizes for each of this versions (non-retina display and @2x retina display).
I'd appreciate some tips from experienced developers regarding this issue and App Store submission: is it required to have two sizes for each image ("normal" one and retina - @2x)? I've been searching in the iOS Human Interface Guidelines and I'm not sure if it is a requirement or just a recommendation. Is there something I could do, in general, to minimize the size of the artwork in an app?
Thanks a lot