DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.
GIMP - GIMP is a multiplatform photo manipulation tool.
Darktable - darktable is an open source photography workflow application and raw developer.
Pixlr - Pixlr® makes it easy to transform everyday images into stunning works of art.
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
Photoscape - high quality photo editing software, that enables you to fix and enhance photos.