How to Use This Tool
Choose an image and pick what should sit behind any transparent areas. Everything happens in your browser — the file is never uploaded.
Why transparency turns black
JPEG has no alpha channel. There is nowhere in the format to record that a pixel was see-through, so every transparent pixel must become an opaque colour during conversion.
A browser canvas starts out as transparent black: alpha zero, and red, green and blue all zero as well. Drawing a PNG onto it leaves those untouched pixels transparent, and encoding to JPEG discards the alpha and keeps what is underneath — black.
Filling the canvas with a colour before drawing fixes it completely. That is what the colour picker above does, and it is why a converter that does not offer one produces black boxes.
Choosing the colour
Match the background the image will sit on. A logo destined for a white page wants white; one going onto a dark slide wants that dark colour.
Getting it wrong is visible in a specific way: anti-aliased edges were blended against transparency, so they contain semi-transparent pixels. Flatten them onto the wrong colour and the edge keeps a faint halo of it — a white-matted logo on a dark page has a pale fringe that no amount of resizing removes.
If the destination is unknown, keep the PNG.
When JPEG is the wrong answer
JPEG discards information the eye is poor at noticing, which works beautifully on photographs and badly on sharp edges. Around text, line art or a screenshot it produces coloured fringing and blocky artefacts that no quality setting fully removes, because the compression works on 8×8 blocks and a hard edge crosses them.
PNG compresses flat colour exactly and extremely well, so a screenshot saved as JPEG is frequently both larger and visibly worse than the original. If the size comparison above shows the JPEG growing, that is the format telling you it is the wrong one.
WebP is offered as an alternative because it does both: lossy like JPEG, lossless when it needs to be, and it keeps an alpha channel. It is supported by every current browser and by fewer desktop applications, which is the only real reason to still choose JPEG.
What quality actually means
The quality number is not a percentage of anything. It selects how aggressively the encoder rounds the frequency coefficients in each block, and the relationship to file size is steep and non-linear.
Between roughly 0.8 and 0.95 most photographs look identical while the file grows substantially, so 0.85 is a reasonable default. Below about 0.6 the block structure becomes visible in flat areas such as skies.
One thing worth knowing: re-encoding a JPEG loses quality again even at quality 1.0, because the original rounding cannot be undone. Always convert from the original PNG rather than from a JPEG you made earlier.