ACX and Audible are the strictest of the audiobook platforms. The cover must be square, and 2400x2400 pixels is the size both the ACX submission guide and Audible's own guidance converge on. Formats in practice are JPEG or PNG. The failure mode is always the same: an author exports a beautiful cover at 4000x4000 or larger, the upload form rejects it or silently degrades it, and the storefront thumbnail ends up soft.\n\nThe fix is mechanical. Resize to exactly 2400x2400, then compress to a sensible size with quality high enough that the title and author name stay crisp. That is it. Skwish runs both steps in the browser, and batching means a full series of ten books is one operation rather than ten. Nothing is uploaded, so pre-release covers stay private.