You could download the photo you would like to make it as the screen saver in PNG format and put it into your Boox, then you could long press the photo on your Boox to choose the “Set as screen” button as standby screen or power off screen.
Here are some demonstrations for standby screen.
You could choose 3 pictures as standby screensavers. Every time your device is in standby mode, it would change pictures from screensaver 1 to 3 in order.
If you just set your own picture as one of screensaver 1, 2, 3, the rest two are by default. If you set two on your own, the rest one is the default one. By setting 3 times with the same picture as screensaver 1 & 2 & 3, you would get the only one screensaver.