PNG vs JPG

Photographs to JPG. Everything with hard edges — screenshots, logos, icons, charts, diagrams, text, line art — to PNG. That single sentence answers the question correctly for the overwhelming majority of images anyone will ever save, and everything below is an explanation of why.

The reason is that the two formats were designed for opposite kinds of picture. JPEG assumes smooth gradual change, which is what a photograph is made of, and it exploits that to throw away detail nobody will miss. PNG assumes exactness matters, stores every pixel, and pays for it in size — which is a bad trade for a sunset and the right one for a screenshot of a spreadsheet.

It is worth seeing once. JPEG compresses in blocks of eight by eight pixels and describes each block as a sum of smooth waves. A photograph fits that model. A hard black line on white does not — describing an abrupt edge with smooth waves leaves a ripple, so text acquires a grey halo, thin lines blur, and flat backgrounds develop faint blotches around anything dark.

At high quality settings it is subtle and it is there. On a screenshot of code, an interface mock-up or a chart with fine gridlines, the difference against a PNG of the same image is immediate, and it does not go away by raising the quality — it only becomes a larger file with slightly less of it.

Nothing bad, and nothing useful. It stores the photograph exactly, which sounds like the better option until you look at the size: a picture that is 800 kilobytes as a JPG is routinely four or five megabytes as a PNG, for a result that is visually identical at normal viewing size.

Photographs have no runs of identical pixels for lossless compression to exploit — every pixel differs slightly from its neighbour, which is precisely what makes them photographs. So a PNG of a photo is close to the raw data, and the four extra megabytes buy detail that a viewer cannot see and a JPEG at high quality had not thrown away.

JPEG has no transparency at all. Not a limited kind, none — every pixel is opaque, and there is no setting anywhere that changes it. A PNG logo with a transparent background converted to JPG comes back with a solid background, usually white, and nothing recovers it afterwards.

That is why logos, icons, product cut-outs and any graphic that has to sit on a coloured background are PNGs. It is also the single most common reason a conversion is regretted: the transparency was the point, and it was silently replaced by white somewhere in the pipeline.

Every time a JPG is opened, changed and saved, it is compressed again from an already compressed source. Crop, save, adjust, save, crop, save — after a handful of rounds the artefacts are visible even in a photograph, and this is how a picture that has been through a chat app, an email and two edits ends up looking like it was photographed off a screen.

PNG has no such behaviour. It can be saved a thousand times and the thousandth is identical to the first. So the working copy of anything being edited should be PNG or a real editing format, with JPG produced once at the end. That is the same discipline as recording audio in WAV and delivering MP3, and for exactly the same reason.

On a website, both are outclassed. WebP does what JPG does at about a third less size and what PNG does with transparency intact, and every current browser reads it. AVIF goes further on photographs. If the image is going onto a web page, the modern answer is to serve one of those with a JPG or PNG fallback, not to choose between the two.

The reason this comparison still matters is everything outside a browser: email attachments, documents, printing, upload forms, phone galleries, older software, and handing a file to a colleague. There JPG and PNG remain the two formats that always work, and the rule at the top of this page is the one to follow.

A screenshot containing a photograph — a design mock-up, a slide with an image on it. The text wants PNG and the photograph wants JPG. Choose PNG: blurred text is more noticeable than a slightly larger file, and it is the part people will read.

A photograph with a transparent cut-out. Only PNG or WebP can do it, so the size has to be accepted, and WebP is much the better choice where it is available.

And a photograph you were handed as a PNG. Converting it to JPG will cut the size by eighty per cent or more with no visible loss, and it is one of the most worthwhile conversions there is — as long as the PNG was not the master copy of something still being worked on.