@j.p.s: It's true in two different ways:
* No matter what you feed it, when Kobo displays an image, it will be dithered. On newer devices, by the hardware, on older devices by the kernel and/or nickel.
* The Calibre Kobo driver can pre-process the book's cover, but only the copy that's used for the device's sleep screen (which will then again go through the dithering pass I mentioned earlier when it gets displayed by the device). Those files are accessible by the user, which is what I think mattered here for Jon, because it meant someone could get their hands on a ready-to-use properly dithered image.
As Is mentioned in an edit, that's an extremely roundabout way to achieve that end result, if you have access to ImageMagick
.
(Momentous event: I think I'm agreeing with Jon on something
).