Free Online Image OptimizerMaking a PNG from a JPEG
This is the one conversion here that reliably produces a larger file than it started with, and the page says so rather than burying it. There are good reasons to want it anyway. There is also a widespread misunderstanding about what it achieves, which is worth clearing up before you press anything.
The file will grow, and that is the correct outcome
Expect several times the original weight. A photograph that arrived at 400 kilobytes can leave at two megabytes or more, and nothing has gone wrong when it does. You have asked for a format that records every pixel exactly, applied to content a lossy format had already compressed hard, so the growth is arithmetic rather than a fault.
Because of that, JPG to PNG is not a way to make anything smaller. If shrinking is the goal you have arrived at the wrong page and the front page is the right one.
What a lossless copy preserves
Here is the misunderstanding. Converting to PNG does not restore detail the JPEG discarded when it was first written. Those pixels are gone and no format can reason them back. What you get is a faithful record of the image as it currently stands, blocking and softness included.
That is still worth having in one specific circumstance: further editing. Every time a JPEG is opened, altered and saved again it is re-compressed, and the damage compounds. Move to PNG first and each subsequent save is exact. Do the work, then export a JPEG once at the end if that is what you need to send.
The other reasons, which are about compatibility
Some software simply demands PNG and will not be argued with: certain print workflows, a few content systems, the occasional icon pipeline. Converting is the shortest route past that and there is nothing more to it.
The other case is transparency you intend to add. JPEG cannot hold any, so a picture destined to have its background cut out has to become PNG or WebP before that work can begin. Note the direction of travel — this gives the file somewhere to put transparency later, it does not create any now.
Worth knowing
WebP is usually the better answer
If what you actually want is lossless quality without the enormous file, WebP does both and every current browser reads it. It is worth a look before committing to PNG for anything headed to a web page.
A note on screenshots
If the JPEG in your hand is a screenshot of text or an interface, converting now cannot undo the harm already done to those edges. Capture as PNG next time instead; the difference on typography is not subtle.