The easiest way to change the standby image is by using these firmwares' default image viewer app which is called "Images" on the apps list.
Open the factory app "
Images", go to the directory in which you placed the image you wanna set as the standby image, open the file, tap on the opened image for the top menu to appear, then tap on the button with the tablet icon:
P.S. - In case you decided to do this by other methods (such as the screenshots directory or by replacing the .zip files in the root directories of the software that contain standby/poweroff pics) and had not succeeded, maybe doing this would help:
Go to
Settings > User settings, tap on "
Standby picture (Restore default Settings)" and select Yes when the device asks if you're sure of this. I guess by doing this, the OS reads the default files/directories that are supposed to contain standby pic(s) again and replaces them with the one(s) currently in its cache or RAM or whatever!