Ok, so I found a quick and easy solution to my problem. It could go better but I do not want to sit around optimizing screensavers for a pocketbook all days
I found the screen savers (plus the identifications from which pictures they were taken;-) online
https://markfiend.wordpress.com/2011...es-identified/
Then I took Irfanview and batch converted them (all at once) simply to bmp files (which will be also greyscale and 800x600).
Load them onto a pocketbook 912 (or 903,etc) into the logo folder and they will be displayed in the center and as big as possible. Due to other page size ratio of the kindles there will be a little space on the upside and downside but I still do like it since most of the backgrounds look still nice since the are to designed to fade in the white background color.