For giggles, lets compare your images to the ones created by:
I used the first color image as the source image and processed it through each respective app at DX and K3 sizes and with various settings. Mangle has an option to turn off dithering but it turns out to just disable conversion to grayscale. Canti defaults to 256 colors (grayscale I think) but you can also set to a lower amount such as 16 in my tests.
The Kindle viewer does support dithering but I turn it off since it slows down the viewing of images. You should also make sure all your images fill up the resolution of the screen since viewing images of different sizes will result in annoying after-images. The add border option in these apps resolves that.
I took all four K3-size images + your Dispersed 8x8 image and viewed them on my K3, full screen and with dithering disabled.
- Mangle_K3_NoDither: Outlines of characters are black and sharp. Obvious color banding on face. Sky is grayish and smoke is lightly visible. Castle is detailed. Text is sharper.
- Mangle_K3: Outlines are grayer and overall image has a blurrier look. Face has smoother color gradients but overall is darker. Sky seems whiter but could be an illusion since the smoke is now darker due to dithering. Castle is gray-filled with little detail. Text is good.
- Canti_K3_256: Similar to Mangle_K3_NoDither, but outlines are slightly less dark, grandients on face is better but complexion is a little darker, sky is similar but smoke is darker, and castle is detailed but a little less sharp. Text is slightly blurry.
- Canti_K3_16: take the canti_k3_256 image and make it darker / have less contrast. Face gradients is okay but probably masked by the overall darkness. Sky is much grayer and smoke is hardly visible. Text is slightly blurry.
- CurstPriest's Dispersed 8x8: viewed it on the K3 without downsizing it beforehand. Similar to Mangle_NoDither but slightly less contrast. Gradients on face is better but complexion slightly darker. Text is good.
I would rank them as such:
- CurstPriest's Dispersed 8x8
The top three are pretty close and 'better' is subjective and depends on what you favor: good contrast but banding vs good gradients but darker
What is clear is that the default settings for Mangle are pretty bad for color images. It may be tuned for high-contrast black and white manga instead of full color western comics.
For now, I prefer Canti because it's a lightweight app that can process folders of comics at once. The author is trying to add auto-level and auto-sharpening options so maybe it can get better results. My K3's screen is probably too small for me to sweat over the tiny details anyway.