At https://python-visualization.github.io/folium/latest/user_guide/ui_elements/layer_control.html, the example shows the error Could not find data/Mercator_projection_SW.png and the image is not displayed.

Presumably the image file is missing or misplaced.

Not sure why it's a PNG when the linked original source is a JPG? Seems to add a little bit of unneeded complexity.

folium is maintained by volunteers. Can you help making a fix for this issue?

I think so if I can find time to work out file hierarchy.