Converting to PNG does not recover quality
It is worth being blunt about this, because it is the most common misunderstanding. Converting a JPG to PNG does not undo JPEG compression or sharpen a blurry photo. A JPG has already discarded detail permanently when it was saved; PNG simply stores whatever pixels currently exist, artefacts and all, in a lossless container. You end up with a faithful, larger copy of exactly what the JPG already looked like — no better, no worse.
So the reason to convert is never to improve the image. It is to get a format that behaves differently going forward: one that survives repeated editing and re-saving without degrading further, that supports transparency if you plan to cut out a background, or that a specific tool or printer will accept when it refuses JPG.
There is also the plain matter of what a program will accept. Some editors, content systems and upload forms are fussy about formats and will take a PNG where they balk at something else, so converting is occasionally just about clearing a gate rather than changing how the image looks or behaves at all.
Why the PNG comes out larger
Expect the PNG to be bigger, often several times the size of the JPG, and sometimes dramatically so for photographs. That is not a fault in the conversion; it is the defining trade-off between the formats. JPG achieves small files by throwing information away, while PNG keeps every pixel exactly and compresses only losslessly, which cannot match JPG on photographic content.
For flat graphics the gap is much smaller, and PNG can even be competitive. But if your goal is a small file rather than a lossless one, converting a photo from JPG to PNG works against you. The right direction for shrinking a photo is JPG to WebP, or compressing the JPG — not moving to PNG.
A lossless copy for editing
The genuine value of a PNG copy shows up when you are going to edit. Every time a JPG is opened, changed and saved again, it is recompressed and loses a little more quality — a generation-loss effect that accumulates over several edits until the image visibly deteriorates. Working from a PNG avoids that entirely, because PNG saves are lossless and can be repeated indefinitely.
The conversion here is done locally in your browser, so the original JPG never leaves your device, and you can convert several at once and download them together. Once you have the PNGs, edit and re-save them as much as you like, and only flatten back to JPG at the very end if you need a small file to share.
Converting a batch, privately
PNG conversion is lossless, so there is no quality slider to weigh up — the tool simply reproduces the JPG's pixels exactly in a PNG container. What there is instead is throughput: drop a whole set of JPGs and each becomes a PNG in turn, ready to download individually or together as a single ZIP.
All of it runs in your browser, so the JPGs are never uploaded to a server. That keeps private images private, lets the tool work offline once loaded, and removes any per-file size cap beyond what your own device can hold. Your originals stay untouched on disk, so you always keep both the JPGs and the new PNG copies.