From Golden Apple Software:
JPEG Optimizer Pro can compress the file size of JPEG images. The file size of JPEG files can be reduced by changing the quality setting - often with little or no noticeable effect on the image. This can reduce the time it takes web pages to load. JPEG Optimizer can be used to compress single images, or it can scan an entire folder (including sub folders), optimizing all JPEG files. It is possible to preview and adjust the settings for each JPEG in a folder, or to automatically optimize all images using the current settings.