Convert PNG to JPG

Almost everyone doing this conversion is doing it for one reason: the file is too big. An upload limit, an email attachment, a page that loads slowly. JPG is the usual answer and it is usually the right one — but whether it helps at all depends on what your image contains, and that is worth thirty seconds before you convert a folder of them.

JPG was designed for photographs. Its compression works by discarding detail the eye is bad at noticing, which is a superb trade on a picture of a face or a landscape where colours shift gradually. A photograph converted from PNG to JPG routinely loses ninety per cent of its size with no visible change at all. If that is what you have, convert it and stop reading.

Screenshots are the opposite case, and the one where people are disappointed. A screenshot is mostly flat areas of identical colour with hard black-on-white edges — exactly the structure JPG handles worst. The compression smears the boundary between a letter and its background, which shows up as a faint halo around text, and because PNG is already extremely efficient at storing flat colour, the file often barely shrinks. Sometimes it grows.

A screenshot of text should stay a PNG, or become a WebP, which handles flat colour and photographs equally well and is read by every current browser. If it has to be a JPG because something downstream demands one, raise the quality setting to 95 and accept a larger file — that reduces the haloing without removing it.

Transparency is the thing to decide before rather than after. JPG has no alpha channel — the format simply cannot record that a pixel is see-through — so anything transparent has to be filled in with a solid colour. White is the default and it is the right guess most of the time, but a white logo on a transparent background becomes a white logo on white and disappears.

You can set the fill colour in the settings under the result, which covers the case where you know what the image will sit on. If the transparency itself is the point, WebP keeps it and still compresses like JPG, and PNG keeps it at the size you were trying to escape.

This is a one-way door, and the warning is easy to give and easy to ignore. JPG discards information permanently. Converting the result back to PNG does not recover it — it stores the already-damaged picture without further loss, which is not the same thing. Keep the PNG if there is any chance you will edit the image later.

Every additional save as JPG compounds the damage, which is why an image that has been through a few chat apps looks the way it does. Convert once, from the original, and keep the original.

An upscaler returns a PNG because it must not throw away the detail it just added, so a photo enlarged four times comes back at 40 MB or more. Image generators writing PNG produce the same problem at a smaller scale. Nothing is wrong with the file; it is simply far larger than anywhere it needs to go will accept.

JPG is the right answer once the image is finished and only has to be looked at — expect low single-digit megabytes with no visible difference on screen. It is the wrong answer if another pass of upscaling, retouching or compositing is still coming, because each of those would then be working from a copy that has already had detail removed.

The quality control appears once you have a result, so you can convert, look, adjust and convert again without starting over. The default sits at 82: high enough that most people cannot tell, low enough to be worth the trouble. In our own measurements the same image came out at roughly 6 KB at quality 20, 20 KB at 82 and 38 KB at 95 — the last step buys very little for nearly double the size.

All of it happens inside your browser. The image is not uploaded, there is no queue, and the page keeps working with your network switched off.