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A JPG arrives already compressed
JPEG is a lossy format: when the camera or phone saved the photo, it already threw away detail the eye barely notices. Compressing a JPG means doing that again. The browser opens the image and saves it once more, at the quality you pick.
Every round loses a little more, and what is lost does not come back. So the most useful rule on this page is a simple one: compress from the original file, the camera's or the one exported from your editor, not from a copy that already went through a messaging app or was downloaded from a website. A photo recompressed several times arrives with blocky smudges, and another round only makes them worse.
Raising the quality does not bring back what was lost
Saving at quality 95 a JPG that was saved at 70 recovers nothing: the file grows and the image keeps the same flaws. When the result comes out heavier than the original, the list says «already small», and the best move is to keep what you had.
The quality number is not a universal measure either. Each encoder uses its own scale, so 80 in one program is not 80 in another. The reliable way is to compare the result with your own eyes, not the number.
Where the weight really is: width and format
The two biggest savings on a JPG do not come from quality. The first is width: a phone photo is easily 4,000 pixels wide, and the page rarely shows more than 1,200. The tool already starts with a 1,600 px cap; set it to the size the photo will actually appear at.
The second is format. We measured 14 images from two real stores with perceived quality matched: WebP needed 21.8% fewer bytes than JPEG at the median, and came out smaller in 11 of the 14. The method is in the article on next-gen formats. That is why WebP is selected by default.
When the result has to stay a JPG
A marketplace that refuses WebP, a print shop, an e-mail attachment, an old system: pick JPEG under «Save as». Start at 80 and only go up if smudges show in skies, skin or gradients, which are the smooth areas and the first to show compression.
Recompressing also drops the EXIF metadata, including the GPS location many phones write into every photo. For publishing on the web that is usually a plus; keep the original if you need that information.
On a WordPress site, with every new photo
Anyone who publishes photos every week is not going to run each one through here. NimboCDN does it at delivery: the original JPG stays in your library, and each visitor gets the photo at the size their screen draws, in WebP on the free plan or AVIF on the Pro plan. Nothing is rewritten, and deactivating the connector puts everything back as it was.
To see what the photos on your pages weigh today, the image size checker weighs each one and estimates what they would weigh afterwards.
Questions
Are JPG and JPEG the same thing?
Yes, it is the same format. The three-letter extension comes from old systems that did not accept four.
How many times can I compress the same photo?
Ideally once, from the original. Every round on a JPG loses a little more, and the loss adds up.
Why did the photo get bigger?
Because it was already well compressed, or because the quality you picked is higher than the file's. The list marks it «already small»: keep the original or lower the quality.
Is there a limit?
Twenty files at a time, up to 25 MB each. The work happens on your own device, and a phone with twenty large photos open at once runs out of memory.
Are my photos uploaded anywhere?
No. The compression runs in your browser, on your device, and you can check it in the network tab of the developer tools: no request goes out.