ImageMagick is really cool. I've been enjoying it since, toying with images to and fro. Info from www.imagemagick.org reads:
ImageMagick® is a software suite to create, edit, compose, or convert bitmap images. It can read and write images in a variety of formats (over 100) including DPX, EXR, GIF, JPEG, JPEG-2000, PDF, PhotoCD, PNG, Postscript, SVG, and TIFF. Use ImageMagick to resize, flip, mirror, rotate, distort, shear and transform images, adjust image colors, apply various special effects, or draw text, lines, polygons, ellipses and Bézier curves.
command:apt-get install imagemagick
example: reduce all images by 40%:
"cd" to the directory where all the images are and use this command to reduce all images by 40%:
command: mogrify -resize 40% *.*
the above assumes all are image files.
example: resize to a specific size:
mogrify -resize 1024X800 *.*
mogrify -resize 1024x800 -format jpg *