Introducing Openplanet Map Thumb:
- Export the thumbnail of any Maniaplanet map file.
- Replace the thumbnail with any arbitrary image file.
- Drag'n'drop an image file on the window to replace the thumbnail.
Source code: https://github.com/openplanet-nl/map-thumb
(I'll probably be making a lot more tools to add to a sort of Openplanet unified tool collection, if people are interested!)