Biggest images use bigger memories.
Aspect ratio of iPhone 4S is 3:2, so it can scale to fit the entire screen without stretching images. (Not iPhone/iPod 5, it's using different resolution.)
1920x1080 is not correct for iPhone/iPod 4th gen, you should use 1920 x 1280. Otherwise if you want to save a lot of memory. These resolutions are suggestions:
-720 x 480
-1440 x 960
Low end devices will have problems with vram memory limit, you could use low resolution that might get stable performance.
I recommend you to use CocoonJS to get good performance for almost all mobiles like iOS 4.5+ and Android 2.3+
I know working with multiple resolutions is pain in the ass.