ImageMagick is a library collection for edit, convert, compound, save images in other formats (greater than 100) and much more...
Available for multiple platforms: Windows, Linux and Mac. Is a very useful tool, you can operate over multiple images with only one instruction. For example, if we want to reduce the quality of al the JPG files in a directory to 80% we only have to enter:
mogrify -quality 80 *.jpg
Also, if we want to resize all to a maximum resolution of 800x600:
mogrify -resize 800x600 *.jpg