convert

• Size

$ convert -geometry 640x480 test.jpg test2.jpg

$ convert -geometry 640x480 big/* vga/image-%02d.jpg

• Reference

http://www.imagemagick.org/script/convert.php
http://www.atmarkit.co.jp/flinux/rensai/linuxtips/289convgraph.html
http://www.21linux.com/archives/2006/01/convert.html
http://newcamel.logos.ic.i.u-tokyo.ac.jp/~s1s5/pukiwiki/index.php?Linux%2F%A5%B3%A5%DE%A5%F3%A5%C9#z4b57657