ImageMagick [1]. You can do all kinds of really cool tasks with it and a little shell script. It’s a free software hidden gem, for sure.

[1] https://imagemagick.org/script/index.php

Not willing to start a flamewar, but there is also Graphics Magick [0], a fork that, for example, adds multi-threading.

[0] http://www.graphicsmagick.org/

ImageMagick has multi-threading, OpenCL support, and even heterogeneous distributed computing support.