[frostschutz]

Just looked at the Kindle preview of your book, it seems the image uses very light shade of grey. The biggest part of it is like #eee which is just one shade away from white #fff. The darkest part of the image is #999 but that's like just one pixel in the center of the image, everything else is considerably lighter. So it's a case of next to no contrast, thus turning it nigh invisible on an eInk screen.

For better results you should increase contrast considerably. The following is original vs. Gimp Colors -> Levels -> Auto. Note this feature only works if the original image is not using the full color spectrum... adding a single fully black pixel to the original would not change the image any. In that case you have to play with the contrast controls... or go to the extreme with Threshold but that only gives good results for images that are supposed to be lineart.

The original image should be nigh invisible on eInk, the modified one should be normal.