KevinShort explains how to replace the power off image somewhere in the developer's corner. You can find my take of it in the
kobo start menu. The relevant scripts are
Code:
mnt\onboard\.kobo\kbmenu\replacements\poweroff
mnt\onboard\.kobo\kbmenu\onstart\poweroff.sh
mnt\onboard\.kobo\kbmenu\scripts\activate\x_install_poweroff.sh
Modify them according to your needs, and adjust them to work without the start menu environment.
For replacing the sleep screen image see
this short thread and the posts linked there.