[kjell]

vips is another great industrial-grade image processing library

https://github.com/jcupitt/libvips http://www.vips.ecs.soton.ac.uk/index.php?title=Libvips

[pbowyer]

And let's not forget NetPBM either, which (when I was doing this a decade ago) was faster and more memory efficient than (Image|Graphics)Magick.