What changes when a PNG becomes a JPG
A photo saved as JPG is usually several times smaller than the same photo as PNG, which is why forms, email and many sites prefer it. JPG cannot be see-through, so transparent parts of a logo or cut-out take the background color you choose; keep PNG or choose WebP when the transparency matters.
Screenshots and text
JPG compresses in small blocks, so sharp edges such as text in a screenshot can show faint smudges at lower quality. Keep the quality at 90 or above for those, or keep the screenshot as PNG, which stores text exactly.
The new files are made in your browser from your images. Nothing is uploaded, and location and camera details (EXIF) are left out.