PRACTICAL GUIDE
Does Compressing an Image Lose Quality?
What lossy and lossless compression actually do to a picture, how much quality you can give up before anyone notices, and which choices are irreversible.
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It depends on which kind of compression
Lossless compression — PNG, and WebP in its lossless mode — stores the same pixels using fewer bytes. Decode the result and you get back the exact image you started with. Nothing is lost, and the savings are modest.
Lossy compression — JPEG, and WebP in its normal mode — permanently discards information the format judges you are unlikely to see. The savings are far larger, and the discarded detail is gone for good.
What a JPEG actually throws away
JPEG separates brightness from colour and stores colour at lower resolution, because human vision is much more sensitive to changes in brightness than in hue. It then breaks the image into blocks and rounds off the fine, high-frequency detail inside each one. The lower the quality setting, the more aggressive that rounding.
This is why the visible symptoms are predictable: soft blocky patches in smooth gradients like skies, and coloured fringing along hard edges such as text or a logo on a flat background.
How much can you give up before it shows
For photographs, quality settings in the region of 75 to 85 percent are the usual sweet spot: the file is typically a fraction of the size of a maximum-quality export, and the difference is hard to see at normal viewing size. Below about 60 percent, artefacts start to become obvious in flat areas. Above 90 percent, the file grows quickly for very little visible gain.
Re-saving is what really hurts
A single lossy save at a sensible quality is rarely a problem. The damage comes from repetition: open, edit, save, send, someone else re-saves it, a platform re-encodes it again. Each pass compresses artefacts from the previous pass as though they were real detail. Always go back to the highest-quality original you have rather than compressing a file that has already been compressed.
Judge it at the size people will actually see it
A common mistake is to zoom to 300 percent, spot the artefacts, and conclude the compression was too harsh. Nobody views your image that way. Compare the original and the compressed version side by side at the dimensions they will be displayed at — on a web page, in an email, on a phone screen — and only then decide whether you have gone too far.
Resizing is not the same as compressing
Reducing the pixel dimensions also loses information, permanently, but it does so in a way that looks clean rather than blotchy. If an image is far larger than it needs to be, resizing first and compressing gently afterwards nearly always looks better than keeping the full size and compressing hard.
Frequently asked questions
Can I restore an image after compressing it?
Not from the compressed file. Lossy compression discards data permanently, and no tool can reconstruct what was thrown away. The only way back is a copy of the original, which is why it is worth keeping one.
Is PNG always better quality than JPG?
PNG is lossless, so it preserves every pixel exactly, but that is not the same as being the better choice. For a photograph, a PNG is typically several times larger than a visually indistinguishable JPEG. PNG earns its place with transparency, screenshots, text and sharp-edged graphics.
Does compressing an image change its dimensions?
No. Compression changes how the pixels are stored, not how many there are. If you want fewer pixels, that is resizing, and it is a separate operation.
Why does my image look worse after uploading it somewhere?
Most social platforms and messaging apps re-encode uploads to their own settings, often after resizing them. That is an extra lossy pass on top of yours, which is one more reason not to compress aggressively before uploading.