PNG files are often far larger than they need to be. A screenshot of a photograph, a chart exported at full resolution, a scanned page saved by a phone app — all of these are stored losslessly by PNG even though the content is photographic and would compress to a fraction of the size as JPEG. Converting them typically cuts the file to between a fifth and a tenth of its original size with no visible difference.
The conversion decodes each PNG, paints it onto a white canvas and encodes the result as JPEG. Where the browser supports WebAssembly workers, a build of MozJPEG does the encoding, which reaches a given visual quality in fewer bytes than the browser’s built-in encoder. High quality uses a factor of 0.92, Normal uses 0.85.
Everything runs locally in the page. Whatever the screenshots contain — internal tools, private messages, account details — none of it is transmitted.
JPEG has no alpha channel, so transparency has to resolve to something. Every image here is painted onto a solid white background before encoding. A logo with a transparent surround therefore comes out on a white rectangle, which is usually what you want for documents and email but is wrong if the logo has to sit on a coloured page.
If you need transparency preserved, stay with PNG, or convert to WebP instead — WebP supports an alpha channel and still compresses far better than PNG.
Keep the PNG when the image is a screenshot of an interface, a diagram, a chart with thin gridlines, or anything containing small text. JPEG’s compression works by discarding high-frequency detail, which is precisely what sharp edges are made of — the result is faint grey shadowing around lettering that becomes obvious when the image is zoomed. For those, use Compress Image instead, which reduces PNG size by scaling rather than by lossy encoding.
For photographic content, typically a fifth to a tenth of the PNG size. For flat-colour graphics and screenshots the saving is much smaller, and the quality cost is much more visible — those are better left as PNG.
They become white. JPEG cannot store an alpha channel, so every image is composited onto a white background before encoding. Convert to WebP instead if transparency has to survive.
On High quality, usually yes, but JPEG always softens sharp edges slightly and small text can pick up faint grey halos. For screenshots with fine type, keeping PNG is the better answer.
High encodes at JPEG quality 0.92, Normal at 0.85. Normal produces distinctly smaller files and is a reasonable choice for photographs, less so for anything with hard edges.
No. Conversion keeps the original pixel dimensions and changes only the encoding. Use Resize Image if you also need the picture made smaller.
No. Decoding and encoding both happen inside your browser tab, so the images never leave your device.
