ImageMagick

Thumbnails

convert -size 800x600 Big.jpg -resize 100 small.jpg 

for file in *.jpg; do convert -size 1024 $file -resize 300 sm-$file; done

Animated GIFs

convert -delay 20 frame*.jpg animation.gif

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Also see: Converter

In Debian: apt-get install imagemagick

Windows: Installing Imagemagick on Windows]