Lossy vs lossless image compression, explained

Lossy compression rebuilds a picture that looks the same to you while discarding data the encoder decides you will not notice — faint noise, tiny colour shifts, detail in shadows. That is how a camera JPEG can be a few megabytes and still look sharp on a phone. The trade is real: the discarded data is gone. Open the file later and you cannot get it back.

Lossless compression stores a recipe for every pixel. Decode it and you get the original grid back, exactly. The file only shrinks when neighbouring pixels repeat — solid colour, text, UI, screenshots. A photograph has almost none of that repetition, so lossless barely helps.

The format is the decision, not a slider setting:

• JPEG is lossy only. It is the universal photo format. A quality around 75–82 is usually indistinguishable on screen.
• PNG is lossless and supports transparency. It is the right home for logos, icons, screenshots and anything that must stay pixel-identical.
• WebP and AVIF can do both. Lossy WebP or AVIF usually beats JPEG at the same visible quality. At the top of the quality slider they can also write a lossless file.

Saving a JPEG as PNG does not restore lost detail. It stores the already-lossy photo losslessly, so the file usually grows. That trap is covered in why PNG files are so large.

A practical rule: photographs go through a lossy format; flat graphics stay lossless.

• Photo for the web, email, or a form. Open the image compressor, keep JPG or switch to WebP, start near quality 80, and read the in → out size on each file. Nudge the slider. There is no target-size field — the listed size is the output.
• Logo, icon, screenshot, UI with transparency. Stay on PNG and compress the PNG. Default PNG recompress is lossless: every pixel, including the alpha channel, stays identical. The file just packs tighter.

Everything runs on your device. Files are never uploaded, EXIF is stripped on export, and a batch downloads as a ZIP. You can go offline after the page loads. More in compressing without visible quality loss and the compressing images cluster.

Lossless will not make a photograph small. It will not undo a harsh JPEG. It will not change width and height — that is resizing, a different job explained in resizing vs compressing.

If you still need to edit the photo later, keep the original and compress a copy. Lossy stacking — saving a JPEG, then compressing that JPEG again — adds artefacts each time. One pass from the original is the honest loop.