Q:

How base image size should I prepare for iOS app?

I'm an iOS developer and usually use Swift. Recently I wonder how base image size should I use for apps in iOS.  
Now I prepare images based on 640x1156 (iPhone SE size) for all apps (For example, I use the background image which is 640x1156). But some people say I should use 750x1334 (iPhone 6,7,8 size) because if use images in iPhone SE size, these quality looks a little low. In addition, only few people use iPhone SE.  
Which size image should I use?
UPDATE
I use background Image and other objects (the 6 squares) and it look different depends on the device, when use iPhone 6s and iPhone X.  

class Constants {

    //width of base design size
    static let guiPartsWidthOnDesign = CGFloat(750.0)

    //ratio for layout
    static var guiPartsMultiplier: CGFloat = UIScreen.main.bounds.width / (Constants.guiPartsWidthOnDesign / 2.0)

    //detect safe area
    static let safeArea = UIApplication.shared.delegate?.window??.safeAreaInsets.bottom

}

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

    self.view.frame = CGRect(x: 0,
                             y: 0,
                             width: self.view.frame.size.width,
                             height: self.view.frame.size.height)

        //backgrond
        let background = UIImageView.init(frame: UIScreen.main.bounds)
        background.center.x = self.view.frame.size.width/2
        background.image = UIImage(named:"BG")
        background.contentMode = UIViewContentMode.scaleAspectFill
        self.view.insertSubview(background, at: 0)

        //downleft blue button
        let blueButton = UIImageView.init(frame: CGRect(
            x: 64/2*Constants.guiPartsMultiplier,
            y: self.view.frame.size.height-(52/2+34/2)*Constants.guiPartsMultiplier-Constants.safeArea!,
            width: 60/2*Constants.guiPartsMultiplier,
            height: 52/2*Constants.guiPartsMultiplier))
        self.view.addSubview(blueButton)
        blueButton.backgroundColor = UIColor.blue

        //green button
        let greenButton = UIImageView.init(frame: CGRect(
            x: (60/2+64/2+128/2)*Constants.guiPartsMultiplier,
            y: self.view.frame.size.height-(52/2+34/2)*Constants.guiPartsMultiplier-Constants.safeArea!,
            width: 60/2*Constants.guiPartsMultiplier,
            height: 52/2*Constants.guiPartsMultiplier))
        self.view.addSubview(greenButton)
        greenButton.backgroundColor = UIColor.green

        //cyan button
        let cyanButton = UIImageView.init(frame: CGRect(
            x: (64/2+(60/2)*2+(128/2*2))*Constants.guiPartsMultiplier,
            y: self.view.frame.size.height-(52/2+34/2)*Constants.guiPartsMultiplier-Constants.safeArea!,
            width: 60/2*Constants.guiPartsMultiplier,
            height: 52/2*Constants.guiPartsMultiplier))
        self.view.addSubview(cyanButton)
        cyanButton.backgroundColor = UIColor.cyan

        //4 blue buttons
        for i in 1..<5 {
            let subBlueButton = UIImageView.init(frame: CGRect(
                x: 64/2*Constants.guiPartsMultiplier,
                y: self.view.frame.size.height-((43.0+CGFloat(50*i))*Constants.guiPartsMultiplier)-Constants.safeArea!,
                width: 60/2*Constants.guiPartsMultiplier,
                height: 52/2*Constants.guiPartsMultiplier))
            self.view.addSubview(subBlueButton)
            subBlueButton.alpha = 1.0
            subBlueButton.backgroundColor = UIColor.blue
        }

        //down bar
        let bar = UIImageView.init(frame: CGRect(
            x:0,
            y: self.view.frame.size.height-50,
            width: self.view.frame.size.width,
            height: 50))
        bar.alpha = 0.3
        bar.backgroundColor = UIColor.blue
        self.view.addSubview(bar)
}

UPDATE2
My assets are:  

UPDATE3
Caring about safe area, it looks like this on iPhone X and it's completely different to the one on iPhone 6s....  

A:

You can check apple developer sites link: https://developer.apple.com/design/human-interface-guidelines/ios/icons-and-images/image-size-and-resolution/
And from Quora blog: https://www.quora.com/When-designing-a-mobile-app-for-what-screen-size-and-resolution-should-I-create-my-designs
And from another: https://blog.fluidui.com/designing-for-mobile-101-pixels-points-and-resolutions/
May be it will be helping you. 

