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
Also see: Converter
In Debian: apt-get install imagemagick
Windows: Installing Imagemagick on Windows]