The following tutorials explain the concepts and examples with groovy code which is actually excerpts from the demo groovy updater scripts in the demo project.
In practice, you sometimes might want to convert data in other format (not in JSON or not exactly in the same JSON format) to ContentNode to import data. In that case, please see the following page in the Hippo JCR POJO Binding project as an example:
In practice, you sometimes have to create thumbnail images to import gallery image sets. Tutorial: Importing Image Set From Files is an example for this use case. The script uses Gallery Magick Image Processing library to generate thumbnail files. Gallery Magick Image Processing project provides utilities and command classes to manipulate image files.
See Tutorial: Importing Image Set From Files for detail.