It's not the kind of front-end crop tool that you right click on, this crop filter runs behind the scenes, being controlled by either jAlbum's engine or a skin. A skin simply creates an instance of such a filter inside init.bsh and installs it by calling engine.addFilter()
So I can take a copy of your code, modify it and load it in the Plugin folder of the skin?
Do I have to change something in init,bsh?
Just install the filter with engine.addFilter(new NameOfYourFilter()); in init.bsh or refer to it with a "filter1" user variable, for instance filter1=class=NameOfYourFilter param=value param2=value ...
Is there somewhere example code and/or documentation for making such an image filter?
All source code is available under the plugins folder. There is also a .sh and .bat file to compile them there. A filter simply needs to implement the JAFilter interface's filter method.