You can place your own "powered off" .bmp images in:
/mnt/ext1/system/logo/offlogo
To ensure they're the correct format/dimension etc, I usually place images I want to use as "offlogos" into the photo folder, open them on the device via the built-in Gallery app, pinch-to-zoom as required to crop, then take a screenshot. Then just copy the image from the screens directory to offlogo dirctory.
I set a double press of the power-button to take a screenshot from the settings menu for ease.
If you make the setting change
Sonnenfee mentioned above for the power-off logo, but set to random, you'll get a different image from your collection (offlogo dir) with every power off... which is how I like it!
You can also change the standby/sleep logo that appears:
https://www.mobileread.com/forums/sh...d.php?t=355272