Reducing image file size without uploading anywhere
Choose an image and adjust the quality slider to re-compress it as a JPEG using your browser's own image encoder. Lower quality settings produce smaller files at the cost of some visual fidelity - the before/after size comparison updates live, so you can find the sweet spot between file size and image quality for your use case (web pages generally look fine at 70-85% quality for photos).
Why faster-loading images matter beyond just file size
Image weight is one of the biggest contributors to slow page load on most websites - and page speed is both a direct Google ranking factor and a major influence on whether visitors stay or bounce. Optimizing images before uploading them is one of the highest-leverage, lowest-effort ways to improve a page's real-world loading performance.
Why this always re-encodes as JPEG
JPEG compression is what makes quality-adjustable file size reduction possible - PNG uses lossless compression with no quality slider, so converting to JPEG is necessary for this kind of optimization. If your original image needs transparency preserved, converting it to JPEG here isn't suitable, since JPEG has no transparency channel.
Why the size estimate is calculated, not measured directly
The displayed "optimized" size is calculated from the length of the generated image data rather than from an actual saved file - this is an accurate estimate of the real download size, since it reflects the same encoded bytes the browser would write to a file, just computed before the download step itself.