Screensaver (coverimage plugin) on Pocketbook device won't work

[Raymen3d3]

Hello, I'm new to the forum!

First of all, I got a Pocketbook Touch Lux 5 - PB628 device for christmas, and it was a nice upgrade after my 7th gen PW3. I installed Koreader because I was curious, and I only read using that. However, the screensaver function would be nice to have, and I set it up according to the wiki.https://github.com/koreader/koreader/wiki/Coverimage
I use the latest stable version, and tried it on the nightly too. The problem is this: Koreader creates the "cover.bmp" file in correct resolution, but won't render it using grayscale, so it won't be used by the system, instead, the default one. Can anyone help, please?