The actual PNGs are in system/framework/framework-res.apk in assets/images.
Code:
12,104 android-logo-mask.png
88,700 android-logo-shine.png
29,659 charcoal_pen_plate.png
12,142 clock_font.png
29,047 nopower.png
304,690 shutdown-default.png
674,055 shutdown.png
442,873 standby-1.png
297,507 standby-2.png
518,392 standby-3.png
I haven't looked too closely how it all goes together, but I simply wrote my own sleep images into /data/local/assets/images/shutdown.png
I do know that there is some goofyness going on and whether your are running the Onyx unobstructed this location might be elsewhere or something?
Just try throwing some images in that /data/local/assets/images
Also, shutdown was landscape or portrait in different versions.