Do you have lots of images to edit, but don't want to call ImageMagick?
$ gimp --batch -
This launches GIMP's builtin Lisp environment (TinyScheme), starts a REPL to read code from stdin, allowing powerful batch processing tasks.
Other language bindings are available, but Lisp is native. Documentation is available at Help -> Produce Browser in the main window.
Also useful if you just need Lisp, as most desktops already have one installed. Add --no-interface if you don't need GUI.