JPG vs PNG: what changes
A JPG is already compressed, and converting it to PNG does not bring back what compression removed. What you get is a lossless copy of the picture as it looks now, which is useful because saving it again will no longer degrade it.
That is the real reason to do this: editing workflows that re-save the file many times, software that only accepts PNG, or a base image you plan to add transparency to. Expect a larger file, typically several times the size of the JPG for photographs.
How to convert JPG to PNG on any device
Before editing
Convert first, do your retouching on the PNG, and export a JPG again only as the final step. Each re-save of a JPG adds a little more damage; a PNG does not.
Files from an iPhone
iPhones can shoot in HEIC, which is a different format. For JPG photos, open Settings, then Camera, then Formats, and choose Most Compatible.
Different extensions
Files ending in .jpg and .jpeg are both accepted. Because the converter checks what is inside the file, a JPG with a wrong extension still works.
Troubleshooting JPG to PNG
The PNG still shows blocky patches
Those patches were created when the JPG was first compressed. PNG preserves them faithfully; it cannot repair them.
My phone photo was not accepted
The converter reads JPG, PNG, WebP and AVIF. HEIC and HEIF photos are not supported, so switch your camera to the Most Compatible setting or export the photo as a JPG first.
The page slows down or reports a memory error
A PNG of a 24-megapixel photo can reach tens of megabytes. Convert a few pictures at a time, and close other heavy tabs on older devices.
Color profile and metadata
A JPG often carries EXIF details such as camera model, exposure and GPS position, plus an embedded color profile. The PNG made here keeps the picture only, so those details are not copied. Photos taken upside down or sideways are rotated to match how they were meant to be viewed.