JPG files are much smaller than PNG for photos, and upload forms often ask for JPG only.
Add your PNG images above, check that JPG is selected, and click Convert images. Each file downloads as JPG; several files come in one ZIP.
Quality is set to 92% by default, which looks the same as the original. You can raise it to 100%.
No. The conversion happens inside your browser, so your photos never leave your phone or computer.