@droopy: That only applies to the nickel thumbnails, not the (K)ePub's cover and/or other embedded images.
Nickel itself will grayscale and dither those (at runtime). You could potentially grayscale the JPGs (don't dither 'em yourself though, JPEG is *not* a paletted format, so it'll basically ruin the dithering on encode. Plus, dithering would also actively harm the compression ratio here).
Depending on content, properly dithered & encoded PNGs might even end up being smaller than a grayscale JPG, though, but that's
another kettle of fish...