PNG to JPG

JPEG has no alpha channel. It cannot store transparency in any form. So when a transparent PNG becomes a JPEG, every transparent pixel has to become some actual colour.

Most tools fill them with black without mentioning it, which is why logos exported to JPEG so often arrive inside an unwanted dark box. This page fills them with white by default and gives you a switch to control it, because the choice belongs to you rather than to an encoder's default.

If the transparency matters, do not convert to JPEG at all. Keep the PNG, or use WebP, which is smaller than PNG and keeps the alpha channel intact.

Converting PNG to JPEG is worth it when the content is photographic. A photograph stored as PNG is commonly five to ten times larger than the same image as a quality-82 JPEG, for no visible benefit whatsoever — PNG simply has no way to exploit the noise and gradient that make up a photograph.

It is the wrong move for screenshots, logos, diagrams and anything containing text. Those have hard edges, and lossy encoding puts visible haloes around them. Chroma subsampling makes coloured text worse still. For that content the PNG is usually smaller as well as sharper.

JPEG has no alpha channel. It cannot store transparency in any form. When a transparent PNG is converted to JPEG the transparent pixels have to become something, and most tools silently fill them with black — which is why a logo exported to JPEG so often arrives with an unwanted dark box around it.

If your image has transparency you have three sensible options: keep it as PNG, use WebP (which supports alpha and is typically 25–35% smaller than PNG for the same image), or flatten it onto a known background colour deliberately rather than letting an encoder choose for you.

PNG's own compression is lossless and works best on images with few distinct colours and large uniform regions. A screenshot of a text document compresses beautifully. A photograph saved as PNG does not, because virtually every pixel differs from its neighbour and there is no redundancy for the algorithm to exploit.

Lossless compression rewrites the file so it takes up less space while every pixel value survives exactly. Decode the result and you get the original back, byte for byte. PNG, GIF and lossless WebP work this way, and so does a ZIP archive.

Lossy compression throws information away on purpose. JPEG converts the image to a frequency representation, then discards the high-frequency detail your eye is least likely to miss. The result cannot be reversed: the discarded detail is gone. That is also why lossy formats reach file sizes lossless ones cannot approach — a photo saved as PNG is routinely five to ten times larger than the same photo saved as JPEG at a sensible quality.

Neither one is better. Photographs, with their smooth gradients and noise, suit lossy encoding. Screenshots, logos, line art, charts and anything with large flat areas and hard edges suit lossless encoding, because lossy encoders produce visible haloes around sharp boundaries.