Transparent PNG to JPG background comes down to empty pixels
Every pixel in a PNG carries a color and one more thing: a note that says nothing is here. JPG has no such layer. Each pixel must land on a color. The moment you save a transparent PNG as JPG, the empty area has to be filled. That fill is the only decision this step asks of you; everything else is handled by the tool.
In Shrink file, set the output to JPG and the empty area becomes white, #ffffff by default. That page has no background color picker. You drop the file in, move the quality slider, download, and white is what you get. There is no second place to change it.
JPG holds no transparency. Shrink file fills white; to pick another color, use Make JPG.
White is the default, not a choice
Most people hunt for a color swatch on the shrink page and never find one. That page gives you a quality slider and an output format. The slider runs from 40 to 95, default 78, and it changes how hard the encoder works, not the fill. Pull it down and the file gets smaller; the empty area stays white either way.
For any other color, open Make JPG. That page fills the transparent area with the color you choose, still white by default, with the same 40 to 95 slider. Download and you get a new file this device just produced. The original is still in its folder unless you replace it yourself.
One more page can lay down a fill: Turn and flip. It outputs PNG, and you can pick a background while turning. If all you need is a JPG and no rotation, skip that detour. Fewer steps, fewer ways to get it wrong.
Check whether the transparency is real
Some PNGs look empty but are actually white. The checkerboard exists only in the viewer; the file was flattened long ago. The test is quick: run it through one JPG conversion and look at the blank area. If it was already white, converting changes nothing, so do not spend time agonizing over the shade.
Going the other way does not work. JPG to PNG will not turn a white background transparent; it just stores white pixels as PNG. To actually clear a background, that is Lift the subject: it removes the background and exports a PNG with transparency. The first use downloads a recognition model from the model publisher into this device's cache. The calculation still runs on this device, and that model is not your photo.
Pick the fill by where the picture lands
- White documents, spreadsheets, email bodies: keep white. It is the least likely to look wrong.
- Dark pages or dark posters: match the surrounding color. A white halo is loud.
- Still need transparency: do not convert to JPG. Stay in PNG.
- The other side only accepts JPG and never mentions a color: send white.
What the conversion costs you
JPG re-encodes at the quality you set, and that is lossy. Transparency is gone, and thin edges and shadows tend to break. A logo with a soft drop shadow, a hairline icon, pale text: these show it first. Edges smear, the shadow breaks into segments, and against white it is obvious.
So order matters. If you need both a new fill and a smaller file, set the color first, then the size, and finish in one pass. Converting back and forth re-encodes each time; detail lost on the first pass does not return, and the file can end up larger. In Shrink file the longest edge defaults to 0, meaning width and height stay untouched. Enter a positive number and only images whose long edge exceeds it get pulled down to that number. Small images are never enlarged, and nothing is cropped.
Set the fill and the quality in Make JPG in one go, so you convert once.
Only two things touch the network
When the page opens, that visit is counted by 51.LA. It counts that someone opened a page, not that an image file moved. Images are not uploaded to ReadyIMG, there is no sign-in, and there is no account server receiving pictures. That boundary is written up in How files are handled. The other case is cutout: the first time you use Lift the subject, a recognition model is downloaded. Converting a background does not download it and does not use it to send your photo anywhere.
Three looks before you send
- Is the blank area the color you meant?
- Are the logo and the lettering still crisp at the edges?
- You downloaded a new file. The original is still where it was.
Up to 40 files at once, and one step can run through the whole set before you download a zip. The queue lives only in this session. Refresh or close the page and anything you have not downloaded is gone, so do not treat an undownloaded result as saved.
If nothing is loaded yet, go back to the bench, drop the image in, then open Make JPG and pick the fill. Where the file actually goes is spelled out in How files are handled.