"GraphicsMagick is the swiss army knife of image processing." - http://www.graphicsmagick.org/

This software purports to be more favorable than ImageMagick.

Usage tips

Docs on scripting: http://www.graphicsmagick.org/GraphicsMagick.html

Convert a bunch of DNG files to low-resolution JPEG

The early -size option here is an optimization, but the actual resizing happens in -resize.

SRC_PATH="${HOME}/Pictures/"
DEST_PATH="${HOME}/Desktop/output/"
SIZE="400x400"
find "${SRC_PATH}" -type f -iname '*.dng' -print0 |
xargs -0 -n1 -P"$(nproc 2>/dev/null || echo 2)" \
gm mogrify \
  -verbose \
  -size "${SIZE}" \
  -format jpeg \
  -resize "${SIZE}" \
  -create-directories \
  -output-directory "$DEST_PATH"

Create a contact sheet

gm montage \
  -geometry 400x400+10+10 \
  -background "#222" \
  -tile 8 *.jpg \
  "$HOME/output.jpg"

See Also