ImageMagick

About
ImageMagick®, version 6.2.5, is a free software suite to create, edit, and compose bitmap images. It can read, convert and write images in a large variety of formats. Images can be cropped, colors can be changed, various effects can be applied, images can be rotated and combined, and text, lines, polygons, ellipses and Bézier curves can be added to images and stretched and rotated.

ImageMagick is free software: it is delivered with full source code and can be freely used, copied, modified and distributed. Its license is compatible with the GPL. It runs on all major operating systems.

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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Installing
In Debian: apt-get install imagemagick

Windows: Installing Imagemagick on Windows

Links

 * http://www.imagemagick.org/script/index.php (official)


 * http://www.cit.gu.edu.au/~anthony/graphics/imagick6/ (lots of practical examples)

Also see: Converter