ok, I solved it for my inkpadX, obtaining a result even better than the one I had on my old Kindle DX:
1) I saved around 100 images of my favorite authors, in grayscale
2) using ImageMagick, I converted them to 16 colors, bmp, resized to fit:
Code:
mogrify -resize 1404x1872 -background white -gravity center -extent 1404x1872 -format bmp -colors 16 *
(NB this is the ImageMagicv6 syntax; for v7 I believe it is slightly different)
3) copy the resulting images to /system/logo/offlogo
4) set personalize/power-off logo to random