OpenCV is one of the big image processing libraries. It is written in C, hence everything can bind to it, even Java (there are samples on GitHub)! Also, it is licensed under BSD, so you can probably use it for anything.
Adopted all around the world, OpenCV has more than 47 thousand people of user community and estimated number of downloads exceeding 14 million. Usage ranges from interactive art, to mines inspection, stitching maps on the web or through advanced robotics.