It doesn’t look like Apple updated the docs yet for the new launch image sizes, but you can find them from Xcode. If you add the new launch images as Maddy says, you can view the new sizes in the inspector panel. iOS 8 includes three new images and does not require different images if the Status Bar is hidden on launch. The sizes are:
iOS 8 iPhone Portrait
Retina HD 5.5 1242x2208
Retina HD 4.7 750x1334
iOS 8 Landscape
Retina HD 5.5 2208x1242
You can add them in Xcode or edit the json file in the .xcassets file and drop the images into the package. If you choose to edit the package, add these lines to the top of the file:
{
"extent" : "full-screen",
"idiom" : "iphone",
"subtype" : "736h",
"filename" : "Default-736h@3x.png",
"minimum-system-version" : "8.0",
"orientation" : "portrait",
"scale" : "3x"
},
{
"extent" : "full-screen",
"idiom" : "iphone",
"subtype" : "736h",
"filename" : "Default-Landscape@3x.png",
"minimum-system-version" : "8.0",
"orientation" : "landscape",
"scale" : "3x"
},
{
"extent" : "full-screen",
"idiom" : "iphone",
"subtype" : "667h",
"filename" : "Default-667h@2x.png",
"minimum-system-version" : "8.0",
"orientation" : "portrait",
"scale" : "2x"
}
I kept the naming convention they used before for the portrait files. There was never a landscape option on phones before, so I kept the naming convention for iPads. Of course, you can name them anything you want.
Edit: Note that the only phone that will launch in landscape mode is the iPhone 6 Plus. The rest of the phones will only launch in portrait mode.