Lens Slate is under the GPLv3.
Startup images are documented in Apple's documentation for web apps on iOS. This documentation is out of date as it only accounts for the original iPhone with a 320 × 480 screen resolution. Supporting newer iPhones with different screen resolutions requires multiple images. Furthermore, startup images apparently do not work in the iOS 9.
On iOS at least, in the absence of a startup image, the app starts with a blank white screen, which is actually a quite good match to the app's other screens.
The app also starts quite quickly.
Given these considerations, I have decided that at this moment there is no need for a startup image.