Compress a PNG

A PNG stores up to four numbers for every pixel: red, green, blue and transparency. It then packs them with the same lossless method a ZIP file uses, which is very good at long runs of identical values and poor at anything noisy. A screenshot of a settings window is mostly flat colour and packs down to almost nothing. A photograph has grain and tiny shifts of tone in every pixel, so a photo saved as PNG can easily be five to ten times the size of the same picture as a JPEG.

Under 90, the picture is rebuilt from a palette of at most 256 colours, each with its own transparency, chosen to match this particular image. The lower the quality, the smaller the palette. Logos, charts, diagrams and interface screenshots rarely use more than a few hundred colours in the first place, so they come out looking the same at a third of the size or less.

Smooth gradients are the weak spot. A soft shadow or a sky needs many in-between tones, and with too few of them you see steps, known as banding. If that happens, raise the quality a little, or go to 90 and accept a larger file.

Stay with PNG when the file is going into a design tool, a document or a print job, or when the edges must be exact. For a website, WebP keeps the transparency and is usually much smaller. For a photo with no transparent parts, JPEG or WebP is the right container and PNG is only costing you space. All three are in the Format menu, so you can try one and compare the sizes before downloading.