A PNG from a camera app, a saved social post or an editor may contain a normal photograph. JPEG is a reasonable alternative when the destination accepts it and you want a more manageable file. Start from the largest original you have, rather than a screenshot of that original. A screenshot may already contain softened detail, interface labels and an unwanted border.
Keep PNG when the image is mainly small lettering, a flat logo or a diagram with fine lines. Those edges can develop visible ringing when saved as JPEG. The right choice comes from what is inside the picture and where it will appear, rather than the extension alone. Save a copy of the source before comparing formats.