Why Your Book Cover Looks Worse After You Upload It

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You spend a week on a cover. The title sits exactly where you want it, the colours are right, and the export looks immaculate at full size. Then you upload it, and the thumbnail comes back with a faint coloured halo around every letter. Zoom in and the edges of your typography look like they were drawn with a wet brush.

Nothing is wrong with your design. The damage was done by the file conversion, and it happened before the store ever saw your artwork.

The problem is not compression, exactly

Most people assume the culprit is the quality slider — that the file was squashed too hard. That is part of it, but the larger offender is a setting almost no tool shows you: chroma subsampling.

JPEG stores brightness and colour separately, because human eyes are far more sensitive to brightness than to colour. The format exploits that by keeping less colour information than brightness information. The common setting, 4:2:0, keeps one colour sample for every four pixels.

On a photograph, that is genuinely invisible. Skin, sky and foliage change colour gradually, so averaging colour across small blocks costs you nothing you can see.

A book cover is the opposite kind of image.

Saturated text against a contrasting background is the single worst case for chroma subsampling — and it is exactly what a cover is made of.

Hard edges between two strong colours are precisely where averaging shows. Red text on teal, white type on a dark photograph, a gold foil title on black: the colour information gets smeared across four-pixel blocks while the brightness stays sharp, and your eye reads the result as coloured fringing.

What "quality 100" does not tell you

Here is the part that catches people out. You can export at quality 100 and still get fringing, because quality and chroma subsampling are separate settings. A tool can honour your quality 100 request and still throw away three quarters of the colour data before the quality setting is even applied.

Worse, most free online converters never expose either control. The typical default is around quality 75 at 4:2:0 — and many of them describe the result as "lossless" or "no quality loss" in the marketing copy.

There is a second trap hiding in your browser. The built-in canvas.toBlob('image/jpeg') call that most in-browser converters use always subsamples chroma to 4:2:0, and there is no parameter to turn it off. A tool built on it cannot give you full colour resolution no matter what its interface promises.

The setting you actually want is 4:4:4 — one colour sample for every pixel, nothing averaged. At quality 100 with 4:4:4, the measured error against your source file is under 1/255 per channel, which is comfortably below what any eye or any screen will show you.

Get the dimensions right before you export

The second thing that quietly ruins covers is resolution, and it is worth being precise about how print size is actually determined.

A PDF page is measured in points, not pixels. What sets the physical printed size is pixels ÷ DPI. At 300 DPI:

• 6 × 9 in trim is 1800 × 2700 px
• 5 × 8 in trim is 1500 × 2400 px
• 5.5 × 8.5 in trim is 1650 × 2550 px
• A Kindle ebook cover is 1600 × 2560 px
• An audiobook cover is square at 3000 × 3000 px

Design at the target pixel dimensions from the start. Scaling a 1200 px design up to 1800 px afterwards invents detail that was never there, and no amount of sharpening puts it back.

Transparency is not optional to handle

JPG cannot store transparency. If your artwork has any — a logo layer, a cut-out author photo, a shape with soft edges — those pixels have to be painted with something during conversion.

Most converters default to black, silently. That is how people end up with a dark halo around a cut-out object, or a cover that arrives with an unexpected black border. If your file genuinely uses transparency, choose the matte colour deliberately so it matches the background it will sit against.

Note the distinction: carrying an alpha channel is not the same as using it. Plenty of PNGs exported from design tools have an alpha channel that is fully opaque everywhere, and those convert without any matte decision at all.

Why your JPG might come out bigger than the PNG

This surprises people, and it is not a bug.

Quality 100 with no chroma subsampling stores a great deal of information. For flat artwork, large areas of single colour, or crisp text — which describes most covers — PNG's lossless compression is simply better suited to the content. A maximum-quality JPG of a flat-colour cover can easily exceed the PNG it came from.

That is the trade the setting exists to make. If file size matters more to you than absolute fidelity in a particular case, step down deliberately rather than letting a converter decide for you.

A checklist before you upload

• Design at final pixel dimensions for your trim size at 300 DPI.
• Export at quality 100 with 4:4:4 chroma, not just "maximum" or "best".
• If your artwork uses real transparency, set the matte colour yourself.
• Check the title text at 100% zoom before uploading — fringing shows on letter edges first.
• Also check it at thumbnail size, because that is where most readers meet your book.
• Keep the layered original. Stores change their requirements, and re-exporting beats rebuilding.

Doing it without uploading your work

Nearly every free converter online works by taking your files onto someone else's server. For an unreleased cover, that is a decision worth making consciously rather than by default.

Free Conva Studio does the whole job in your browser: it converts at quality 100 with 4:4:4 chroma using a real MozJPEG encoder compiled to WebAssembly rather than the browser's built-in one, flags files that actually use transparency so you can choose a matte colour, and exports print-ready PDF where pixels ÷ DPI sets an exact page size. It is also a layered cover editor with trim-size presets for the dimensions listed above, so you can design at the right size and export correctly in one place.

No account, no upload, no watermark. Your files never leave your machine — which, for a cover nobody has seen yet, is the point.