Been a while since I made a native app for IOS, this article is to refresh my memory and hopefully it might be useful to someone else. Forgot the screen dimensions but found a great resource here:
The screen sizes are as follows:
So for retina devices, you would multiply the dimensions of the image by 2 or three depending on the display resolution. So for iPhone 4s you would make a retina image the size of 1280 X 1920 etc. So for the iPhone 6+ you would do 3726 X 6624. Apple recommends you use a launch file/storyboard file for more images on iphone 6 and 6+.