SIPS - Easy Image Conversion
SIPS
Add Icon(Mostly Useless)
Resize to Largest Plane - Very Useful - Supports Wildcards:
Convert PDF to JPG
Other Command-Line Conversion Options
Imagemagick
Convert - convert image A to image B
Mogrify
Alter existing images in place
To resize a folder full of .jpgs, use the following examples (adjust -sample and -quality values to your liking):