Cover art is a bad candidate for aggressive compression. A photograph tolerates a low quality setting because the eye cannot tell where the detail went. A cover does not: it is type on a flat background, and the moment quality drops the letterforms blur, gradients band, and the storefront thumbnail — often under 300px wide — turns into mush. So the approach has to be measurement-first rather than maximum-reduction.\n\nIn practice most audiobook cover bloat comes from resolution, not quality. A 4000x4000 PNG or a print-ready export is enormous and almost all of those pixels are discarded when the storefront renders at 2400px or less. Resize first, then compress lightly. That single ordering change usually gets a multi-megabyte cover under a normal upload limit with no visible loss at all.