The 'convert' utility from ImageMagick is often used for this.
http://www.cyberciti.biz/tips/howto-linux-creating-a-image-thumbnails-from-shell-prompt.html has an example that I adapted here.
Given two directories, images and thumbnails /, this small script converts all images into thumbnails in another directory using the thumb. at the beginning of the file name:
#!/bin/bash for i in images/* do echo "Prcoessing $i ..." /usr/bin/convert -thumbnail 200 "$i" thumbnails/thumb.$(basename "$i") done
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