IH-5, v5.1.0 and later, has some major User-Interface (UI) changes.

If you want to add an image to a product using the Image Handler 'tool', you need to first select the 'primary' image ... that's where the 'add' becomes available. I've got documentation on the updated interface available on the GitHub site: https://github.com/DivaVocals/zen_Im...age_handler.md.