@Espoir
I've checked the .png files and they're all valid. Since your book displays fine both with ADE and Apple Books, this appears to be a PocketBook issue.
If device compatibility is important to you, you might want to replace the image files with HTML files because you can create the same look that your book now has by
embedding fonts with Calibre or Sigil.
(You can also use my
CSSFromFonts Sigil plugin to create the font framework.)