@Machinus
Try to convert the image with Image Magick (credit goes to NiLuJe from a an old post from Kobo forum, I believe):
Code:
convert input.png -filter LanczosSharp -resize 1072x1448 -background black -gravity center -extent '1072x1448!' -colorspace Gray -dither Riemersma -remap eink_cmap.gif -quality 75 -define png:color-type=3 png:out.png
First copy the attached file to the directory where you will be using the command prompt. Replace input.png with your image filename.
I converted all my screensavers this way and never had an issue since.