Compress JPEG

Lossless compression rewrites the file so every pixel survives exactly — decode it and you get the original back, byte for byte. PNG, GIF, lossless WebP and ZIP all work this way.

Lossy compression discards detail on purpose. JPEG converts the image to a frequency representation and throws away the high-frequency detail your eye is least likely to miss. That is irreversible, and it is how lossy reaches sizes lossless cannot: a photograph saved as PNG is routinely five to ten times larger than the same photograph as JPEG.

Neither is better. Photographs suit lossy encoding; screenshots, logos and line art suit lossless, because lossy encoders leave haloes around hard edges.

The quality number is not a percentage. It is an index into quantisation tables that control how aggressively detail is discarded, and its relationship to file size is steep and non-linear.

• 95–100 — near-original, often only 20–35% smaller. Rarely worth it.
• 80–90 — photography you care about. 50–70% smaller, no artefact at normal viewing distance.
• 70–80 — ordinary web images. Usually 70–85% smaller.
• 50–70 — visible softening in smooth areas; thumbnails survive it.
• Below 50 — blocking, ringing and colour banding. Placeholders only.

Returns diminish fast at the top: 100 to 90 usually halves the file for no perceptible change, 90 to 80 saves far less. If you remember one number, use 80.

A baseline JPEG is one top-to-bottom pass, so on a slow connection it paints in from the top. A progressive JPEG stores several passes of increasing detail, so the whole image appears at low quality and then sharpens.

Progressive is usually a few per cent smaller and makes a page feel loaded sooner, at slightly higher decode cost. Above roughly 10 KB it is the better default.

A digital image has exactly one physical property: its pixel dimensions. A 3000 × 2000 photograph holds six million pixels whether its metadata claims 72 DPI or 300 DPI.

DPI is a printing instruction in a metadata field. It tells a printer how many pixels to place per inch, which sets the printed size — 3000 pixels at 300 DPI is a 10-inch print, and at 72 DPI a 41-inch print of the same pixels.

So "save at 72 DPI for the web" does not shrink a file. Reducing pixel dimensions does. For A4 at 300 DPI, supply about 2480 × 3508 pixels.

JPEG has no alpha channel and cannot store transparency. Converting a transparent PNG forces those pixels to become something, and most tools fill them with black — which is why a logo exported to JPEG arrives in an unwanted dark box. Keep it as PNG, use WebP (alpha support, typically 25–35% smaller than PNG), or flatten onto a chosen background deliberately.

Colour is the other gap. Screens mix additively in RGB; printing presses mix subtractively in CMYK, and converting between them always approximates. A CMYK JPEG is converted to RGB before a browser can display it, and saturated cyans and deep blues can shift noticeably. Compress and resize in RGB for screens, and keep a separate CMYK master for print.

Every time a JPEG is decoded, edited and saved, the encoder re-runs quantisation on data already quantised. Errors from the previous pass are treated as real detail and collect new errors on top. This is generation loss, and it only accumulates.

Re-saving at quality 90 restores nothing; it spends more bytes preserving the earlier pass's artefacts. Compress from the highest-quality original you have, and settle on one output size in a single pass rather than nudging quality down and re-saving.