If you are referring to launch images, yes, they must be a specific size. The "Launch file or image (required for all apps)" row in the link I posted previously gives the sizes for all iPads and for all iPhones through iPhone 5. Clicking the "Lauch Files" link on that row takes you here:
https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/MobileHIG/LaunchImages.html#//apple_ref/doc/uid/TP40006556-CH22-SW1
The sizes for iPhone 6/+ are listed at the bottom of that page.
Another way to see this information is to go to your Assets.xcassets file (in Xcode), right-click in the left side of the editor area, choose App Icons and Lauch Images, and choose New iOS Launch Image. On the right side, choose the Attributes inspector, and select all the iPad and iPhone devices. Now click on each image place holder in the editor area, and the expected size will be shown at the bottom of the Attributes inspector. Note that instead of "iPhone 6" and "iPhone 6+", it will have "Retina HD 5.5" and "Retina HD 4.7", and instead of "iPhone 5" it will have "Retina 4".
If you have other photos or images (besides launch images) that you want to be full screen at native resolution within your app, use these same sizes.