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To add more confusion:

There is also "netpbm", a collection of classical unix command line tools for image processing. Shell scripts can then automate that.

http://netpbm.sourceforge.net/

Netpbm uses a rather simplistic intermediate file format. This has downsides caused by a lack of features, but also upsides in that it is rather easy to program your own image filter if you want to tinker with exotic custom image processing.