Compress PNG Images

How a PNG is made smaller

A lossless PNG cannot be made smaller without losing information, so compression of a lossless format means a different trade: reducing the number of distinct colours. If an image uses 40,000 colours and a palette of 256 is applied, the file drops dramatically, because each pixel now references a small table. The cost is that colours are quantised to their nearest palette entry, which shows as banding in smooth gradients and a slight shift in subtle colour variation. That is a genuine loss of fidelity traded for size, and it suits a screenshot or a logo far better than a photograph.

NoteLossless PNG cannot shrink without loss. Reducing the palette shrinks it, and that is a real quality trade.

Which images tolerate a reduced palette

An image survives a small palette when it contains few distinct colours to begin with. A logo in two or three colours, a flat illustration, an icon, a chart, and a screenshot with limited shading all reduce dramatically and look essentially unchanged. A photograph is the opposite: it contains a continuous range of skin tones, sky gradients, and foliage detail, and reducing it to 256 colours produces visible banding across every smooth area. The right question is not how small the file should be but what the image is.

• Few colours to begin with: logos, icons, charts, flat illustrations reduce well.
• Screenshots reduce well, though small text can suffer at low palette sizes.
• Photographs band badly. Use a modern lossy format instead.
• Gradients are the first thing to break in any image.

NoteA flat graphic reduces to 256 colours with no visible change. A photograph will band.

Lossless WebP is often the better answer

There is a strictly better option than shrinking the PNG palette, and it involves no quality loss at all. WebP has a lossless mode that is typically smaller than PNG for the same pixels, so a PNG can usually be converted to lossless WebP and come out both lossless and smaller. That beats quantising, because nothing is given up. The one thing to check is whether the destination accepts WebP, since older software and some upload fields do not.

• Convert to WebP lossless for a smaller file with no quality loss.
• That beats reducing the palette, which does cost fidelity.
• Check the destination accepts WebP before converting a whole set.
• Keep PNG when transparency plus universal support is required.

NoteConverting to lossless WebP shrinks a PNG with no quality loss at all. Quantising is not the only option.

When JPG is the right answer instead

If the image is a photograph and the file is simply too large, neither a reduced PNG palette nor a lossless conversion helps much, because the problem is that there is a lot of photographic detail in it. JPG discards that detail, which is exactly what you want for a photograph, and gets the file small. A common and sensible workflow for a heavy screenshot is to keep PNG where text and edges matter, and use JPG where the image is photographic. The mistake is applying one format decision to an entire folder of mixed content.

• A photograph that is too large: convert to JPG.
• A logo or diagram: keep PNG, or move to lossless WebP.
• A screenshot: PNG for text clarity, JPG if the size limit is hard.
• A mixed folder: convert in batches by content type.