ImageOverlay
Overlays an arbitrary Image over the Terrain, for example to recreate a real world city.
The image file has to be “[Steam Skylines Folder]/Files/overlay.png” and supports transparency and up to 4096px*4096px. Don’t forget to put that file there, otherwise you won’t see any effect.
Use the up/down arrow keys to move the image layer up and down. Use the left arrow key to switch between full coverage and 25 tile coverage. Works both in game and in the map editor.