sudo port install ImageMagick will install the ImageMagick open source port for MacBook.
This software has powerfull command line utilities for converting, resizing, croping, in one word to say managing all kind of operations on images.
If you want to make thumbnails, for a list of files in a directory there are few ways. Here is one of them:
ls *.JPG | xargs -I {} convert -thumbnail 128x96 {} _{}
This reads all JPG files and pipes the output to xargs. xargs is another great utility that applies the input from ls to convert
convert is one of the great utility pieces that comes with Image Magick. This utility has the option thumbnail that resizes the given image and make a new thumbnail (in my example the thumbnail has the underscore in the filename (_filename).
Great if you want to make 1000 thumbnails on the fly 
. I just love the shell and the power of shell commands .