Notes:
It was clear that I wouldn´t want to start at zero, reimplementing basic functionallity like reading of image file formats, showing images on the screen, etc. that already exists.
So I searched for a library / toolkit that provides the basic image processing functionallity.
I found that ImageJ was the right basis for the project.
Unlike other frameworks it has been created for the treatment of microscopy images
It provides a solid image processing and analysis framework.
A lot of plugins for specific tasks are available.
It is alive today.
Good documentation is available.
It is public domain and the source code is available, allowing to make changes when necessary.