PNG stores every pixel exactly and supports transparency, which makes it excellent for logos, screenshots and line art but wasteful for photographs. JPG throws away detail the eye is poor at noticing, so a photographic PNG often shrinks by 80 to 90 percent with no visible change. The one thing to watch is transparency: JPG has no alpha channel, so any transparent area has to become a solid colour.
They are filled with white. JPG has no alpha channel, so transparency cannot survive the conversion. If your image has a transparent background and you need to keep it, convert to WebP instead, which supports both compression and alpha.
This happens with flat-colour images such as screenshots, logos and diagrams. PNG compresses large areas of identical colour extremely well, while JPG adds noise around hard edges. If your image is mostly flat colour and sharp text, PNG is genuinely the better format and converting will cost you both size and quality.
No. JPG compression is lossy and permanent. Keep your original PNG if you may need to re-edit later, and treat the JPG as an output for sharing or publishing.
Files up to 25 MB are accepted. The conversion runs on your own device using the browser canvas, so very large images depend on your available memory rather than any server quota.