Unpublished cover art is the one asset an author cannot afford to leak before release day. Upload-based compressors put that file on somebody else's server for however long their retention policy allows — which is a real risk for a cover that is not public yet. Skwish compresses with the browser canvas API, so the file is processed on your machine and only the smaller result is written to disk.\n\nFor audiobook covers specifically, the useful work is usually a combination: get to a square, get under the distributor's practical upload size, and keep text legible at thumbnail scale. A title set in a thin serif is the first thing to smear if quality goes too low, so compress conservatively and check the thumbnail size before you ship.