    PNG (uncompressed, e.g. 89 MB) vs JPG (compressed, e.g. 184 kB)

    GIMP
       compress
       resize
       batch

    ImageMagick
       list formats:
          convert -list format
       convert PNG to JPG:
          convert input.png output.jpg
       convert all PNGs to JPGs:
          mogrify -format jpg *.png
       convert SVG to PNG at 1000 DPI:
          convert -density 1000 -units PixelsPerInch input.svg output.png
       compress JPG to quality 50% width 1000:
          convert input.jpg -quality 50% -resize 1000 output.jpg
       compress all JPGs to quality 50% width 1000:
          mogrify -quality 50% -resize 1000 *.jpg