Does setting JPEG quality to 100 mean no quality is lost?

The top of the slider is not 'lossless'

The quality slider in the compressor controls how much detail the encoder keeps, but it does not turn JPEG or WebP into a lossless format. Even at 100, the image goes through mathematical rounding that nudges pixel values. More importantly, your photo is most likely a JPEG that lost some detail when it was taken, and no setting brings that back.

So why does the file grow? Quality 100 tells the encoder to store everything as precisely as it can, including the small artifacts left by the first compression. You get a much heavier file carrying essentially the same picture.

A test on a product photo

Look at the jump between 95 and 100: five notches on the slider nearly tripled the size. And the visual gap between 80 and 95 is far smaller than those numbers suggest.

What to do instead

• For archiving or later editing: keep the original file as it is and do not run it through any compressor.
• For your store or for sending: use the Image compressor at 70 to 85 and check the preview to confirm the difference is not noticeable.
• For a hero image where you want very high quality: try 88 or 90 and watch the size. You will usually find a point that keeps the look without bloating the file.
• If the tool warns that the file got larger, that is your cue to lower the quality or keep the original.

What about converting to PNG?

PNG stores pixels without loss, but it cannot restore detail a JPEG already lost, and for photographs it produces very heavy files. That is why the compressor does not output PNG at all. If you need a PNG for another reason, say a program that accepts nothing else, use the Image format converter and expect a bigger file, not a smaller one.