ImageMagick is a software suite to create, edit, and compose bitmap images. It can read, convert and write images in a variety of formats (about 100) including GIF, JPEG, JPEG-2000, PNG, PDF, PhotoCD, TIFF, and DPX.
XnConvert is an easy image converter for graphic files, photos and images available on Windows, MacOSX and Linux.
With Filestar you can "do anything with any file". Get rid of repetitive tasks, save time and boost your productivity with over 30.000 file conversions available. Transform or convert hundreds of files, all at the same time with a few simple clicks.
ImBatch is a batch image processor with a nice graphical user interface. It will allow you to edit multiple image files at the click of a button by using scripts. The script will be created by combining Tasks.
With BIMP you can apply a set of manipulations to an entire group of images! It provides an easy to use interface and a lot of batch functions: