OK. But still - simple resize can distort image ratio. After cleaning it should be scaled (not resized) to resolution as close as possible to target device resolution and then all free space around it should be filled with white color.
Process that you performing in conjunction with potentially not very high resolution and probably compressed images can highly reduce quality of image displayed on e-ink screen.
You don't use zoom but it is worth noting: this method blocks any method of high quality zooming as image resolution is too low.
EDIT:
My methods will probably create files bigger than yours. But you might try KCC
Normal quality mode.
EDIT2:
We just represent two schools. You wan't just slap images on Kindle and read them. And I want achieve maximal possible quality