I alsways forget these commands and i always end up having to google them
To resize all images in current directory to 800 width (height will be reduced proportionately) :
mogrify -resize 800 *.jpg
Resize all images to 800 height (width will be reduced proportionately)
mogrify -resize x800 *.jpg
Resize all images to a maximum dimension of 800×800. This means that max of width and height will be considered while resizing :
mogrify -resize 800×800 *.jpg
Reduce the size of images to the percent of original :
mogrify -resize 75% *.jpg
Note that in all above commands the original file will be replaced with the new one. Also the examples given above uses only jpg files but you can surely do it with other image formats as well.
http://blog.coppermine-gallery.net/2009/07/08/batch-resize-using-imagemagick/
Why do they call it MOGRIFY!