Compress an image without losing quality (visibly)

1. Keep the pixels unless the image is far larger than where it will be shown.
2. Use JPEG quality 75 to 85. Above that the bytes buy nothing you can see; below 60 the blocks appear.
3. If you must resize, sharpen afterwards. A downscale without sharpening looks soft, which people misread as compression damage.
4. Strip metadata. EXIF, embedded thumbnails and colour profiles from a camera can add 100 KB and change nothing visible.

The tool applies all four. Use the Compare button and the 1:1 zoom on any result to check for yourself.

If you need mathematically identical pixels (medical, archival, print production), use PNG, lossless WebP or keep the original. Those files are large, and no online tool can make them small without changing pixels. For everything viewed on a screen, the rules above are what matter.