ios will always tries to take the best image, but will fall back to other options .. so if you only have normal images in the app and it needs @2x images it will use the normal images.
if you only put @2x in the project and you open the app on a normal device it will scale the images down to display.
if you target ios7 and ios8 devices and want best quality you would need @2x and @3x for phone and normal and @2x for ipad assets, since there is no non retina phone left and no @3x ipad.
maybe it is better to create the assets in the app from vector graphic... check http://mattgemmell.com/using-pdf-images-in-ios-apps/