THIS IS UNOFFICIAL TRICK TO DEFINE IMAGES FOR DIFFERENT SCREEN SIZES (iPhone XS Max, iPhone XS, iPhone Xr).
I found a trick. In my application we have launch screens for all iPhone Devices (see screenshot).

Next screen which appears after Launch screen
is Login
. It should have the same image like Launch screen
.
Unfortunately Xcode
creates the following image set
in Assets
out of box (iPhone 1x, 2x, 3, iPad 1x, 2x - see image).

And during transitions from Launch Screen
to Login screen
I see artifacts because Image stretching on iPhone X, Xs Max, iPhone Xr
To fix the issue I copied Contents.json
file from LaunchImage.launchimage
folder to BackgroundImage.imageset
folder in Finder (see steps on image below)

UPDATE
the content of AppImages.xcassets/BackgroundImage.imageset/Contents.json
file:
{
"images" : [
{
"idiom" : "iphone",
"scale" : "1x"
},
{
"idiom" : "iphone",
"subtype" : "retina4",
"scale" : "1x"
},
{
"idiom" : "iphone",
"filename" : "retina4640x960.png",
"scale" : "2x"
},
{
"idiom" : "iphone",
"filename" : "retina4640x1136.png",
"subtype" : "retina4",
"scale" : "2x"
},
{
"idiom" : "iphone",
"filename" : "retina47750x1334.png",
"subtype" : "667h",
"scale" : "2x"
},
{
"idiom" : "iphone",
"filename" : "iPhoneXr828x1792.png",
"subtype" : "1792h",
"scale" : "2x"
},
{
"idiom" : "iphone",
"scale" : "3x"
},
{
"idiom" : "iphone",
"subtype" : "retina4",
"scale" : "3x"
},
{
"idiom" : "iphone",
"filename" : "retina551242x2208.png",
"subtype" : "736h",
"scale" : "3x"
},
{
"idiom" : "iphone",
"filename" : "iPhoneX1125x2436.png",
"subtype" : "2436h",
"scale" : "3x"
},
{
"idiom" : "iphone",
"filename" : "iPhoneXSMax1242x2688.png",
"subtype" : "2688h",
"scale" : "3x"
},
{
"idiom" : "ipad",
"filename" : "ipad1024x768.png",
"scale" : "1x"
},
{
"idiom" : "ipad",
"filename" : "ipad2048x1536.png",
"scale" : "2x"
}
],
"info" : {
"version" : 1,
"author" : "xcode"
}
}
And now in Xcode
I see the following templates. I can define background image for the following devices Retina HD 4.7"(iPhone 6, iPhone 7, iPhone 8), Retina HD 5.5" (iPhone 6 Plus, iPhone 7 Plus, iPhone 8 Plus), iPhone Xr, iPhone X/iPhone Xs, iPhone Xs Max
