picprep

From Myndkryme Laboratories:

picprep is a command-line utility for batch processing digital images. It allows for scaling by absolute size, percentage of original size, changing the bitdepth, change the fileformat ( jpg and png only at this point )...It's very useful for processing an entire directory of images with little fuss, great for shrinking those digital photos down to half-size when you want to share them with your friends.