On HiDPI output(scale 2) the background image is just a top left corner of the original image.
This can now be fixed by switching to css instead of using the background option, which is a bit broken due to weirdness around gtk and cairo surfaces...
See https://git.sr.ht/~kennylevinsen/gtkgreet/commit/90424c865f3b8509ec684a987c76599e697fbb83 for examples.