filling screen with image with letterbox

I have set scale = “letterbox” in config.lua and getting black zones at top and bottom of screen.
Is there a way I can fill them with an image?

If you set a display object’s width and height to display.actualContentWidth and display.actualContentHeight then it should cover the whole screen.

I guess it depends on device, but normally with letterbox scaling the black zones would be either on top/bottom or sides, but not on all sides on the same device.