When you are not able to embed/add an image from GitHub to your MarkDown files such as a page in Sphinx or Jupyter Book, maybe you are using blob instead of raw in the image URL.
For example, if you use https://github.com/hci-unihd/plant-seg/raw/assets/images/plantseg_overview.png then the link is to an image, but https://github.com/hci-unihd/plant-seg/blob/assets/images/plantseg_overview.png is to the GitHub page that contains the image.
Click to see the difference:
