What is Zim?
Zim [zim-wiki.org] is a free note-taking application available for Windows, macOS, and Linux. It lets you create and open "wiki" style folders in your PC that store rich text notes in a tree structure, meaning that a note can be a child of another note. It's also called "Zim Desktop Wiki."

Observation: the first time you run Zim it will ask you to "add a notebook." A notebook in Zim is a folder where the notes will be stored. This is just like choosing where to save a file, except it will save the notes as a whole folder full of subfolders inside.

Observation: in Zim, it's possible to embed images and resize them. Embedding is done by copying the image into the notebook's folder, and resizing is a destructive operation, meaning if you resize an image you can't just resize it back to its original size later to get the original image because the pixels will have been modified.
Questions and Answers
Is Zim Open Source?
Yes, Zim is open source licensed under GPL v2. Its repository is found at https://github.com/zim-desktop-wiki/zim-desktop-wiki.