ImageMagick - ImageMagick is a software suite to create, edit, and compose bitmap images.
Ralpha Image Resizer - High-speed image batch conversion tool
OpenCV - OpenCV is the world's biggest computer vision library
ImBatch - ImBatch is a batch image processor with a nice graphical user interface.
Cloudinary - Cloudinary is a cloud-based service for hosting videos and images designed specifically with the needs of web and mobile developers in mind.
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.