ImageMagick

Use ImageMagick to create, edit, compose, and convert digital images. Resize an image, crop it, change its shades and colors, add captions, and more.

Resizing files in a directory

nice -n 19 mogrify -path . -filter Triangle -define filter:support=2 -unsharp 0.25x0.25+8+0.065 -dither None -posterize 136 -quality 82 -define jpeg:fancy-upsampling=off -define png:compression-filter=5 -define png:compression-level=9 -define png:compression-strategy=1 -define png:exclude-chunk=all -interlace none -colorspace sRGB -strip *.jpg

[Source]

#end