Are iOS applications required to have retina images?
No. You could submit an app with purely hand drawn images or 8 bit old school graphics and soundtrack

The only required retina assets are the store icons and app icons as specified in the iOS HIG:


https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/MobileHIG/IconMatrix.html#//apple_ref/doc/uid/TP40006556-CH27-SW1