Q:

Displaying Images in iOS application

I need to display a few images in my IOS Application. What should I do so that the images display appropriately across all devices?
Do I have to set the size of the image manually based on the device? Please clarify. 

A:

You wouldn't use SpriteKit just to display images. You would load the images as UIImage and then create a UIImageView that you can place on the screen wherever and however you want and then you just assign the UIImageView your UIImage. UIImageView has a lot of properties you can set how images are displayed (e.g. if they are scaled and how they are scaled or if they are not scaled, how they shall be aligned within the viewable area, and so on). You can draw a UIImageView on top of a SpriteKit scene, that is no problem on iOS (on iOS everything is drawn by OpenGL ES or Metal anyway).
Of course you can also embed any image as a sprite if you like:
UIImages * img = ...;
SKTexture * tex = [SKTexture textureWithImage:img];
SKSpriteNode * sprite = [[SKSpriteNode alloc] initWithTexture:tex];
// If you don't use ARC, I'd add the following below:
// [sprite autorelease];

Now you can perfectly integrate it into the scene in whatever way you like an perfectly align it will all your other sprites. Yet if you just want to paint an image over the scene:
SKScene * scene = ...;
SKView * sceneView = scene.view;

UIImageView * imgView = [[UIImageView alloc] init];
imgView.image = img;
// Whatever content mode you prefer
imgView.contentMode = UIViewContentModeScaleAspectFit;
// Where shall it be placed and how big shall it be.
imgView.frame = CGRectMake(posX, posY, width, height);
// If you'd use this, it will cover the whole scene: 
// imgView.frame = sceneView.frame;

// Add it on top of your scene
[[sceneView parent] addSubview:imgView];

// If you don't use ARC, don't forget to release it:
// [imgView release];

If you load an UIImage from your application bundle with [UIImage imageNamed:@"blah"] and the image exists in different resolutions for retina devices (blah.png, blah@2.png, blah@3.png), the system will automatically load the image it considers most suitable for the screen of the current device. This is nothing you have to deal with.
If you need to convert between scene coordinates and view coordinates, SKScene offers -convertPointFromView: and -convertPointToView: for this case. If the scene fills the whole screen, then these actually convert between scene and screen coordinates. 
Even if devices have different resolutions, your scene can always have the same "virtual size". So you can always say that the scene is 400x300, no matter what the real screen resolution is. In that case placing a sprite of virtual dimension 200x150 at the virtual coordinates (100,75) will always center it on the screen, no matter what device or how big the screen really is (well, assuming that the SKSceneView really covers exactly the whole screne, of course). The size of a SKScene is just the coordinate system you want to have for layouting your game, it can be whatever you want it to be, it can be bigger or smaller than the real screen. 
The scene is always drawn into a SKSceneView. The size of the SKSceneView is the real size of your scene in screen coordinates. So if you SKScene is 480x320 and the size of the SKSceneView is 1440x960, then moving a sprite one pixel in your scene will in fact move it 3 pixels on the screen. Yet if your SKScene is 1136x640, but your SKSceneView is only 586x320, then moving a sprite two pixels in your scene will only move it one pixel on screen. Your scene is always scaled up or down as required.
Personally I'd either stick with the same size across all devices or maybe just make two or three device classes but not adopt the game for every single device and every existing screen resolution.

