JPG - best for photos; small files, no transparency
PNG - best for graphics, screenshots and anything needing transparency
WEBP - modern format, smaller than both, supported by all current browsers
Steps
Open the converter for your source format (JPG to PNG, PNG to WEBP, and so on), upload the image, and download the converted file. Conversion happens on the server using GD or Imagick.
Things to know
Converting JPG to PNG does not add transparency or recover detail
PNG to JPG flattens transparency onto white and shrinks the file
WEBP usually produces the smallest files at the same visual quality