png to mm conversion is certainly related to export of PNG graphics or its import to for the purpose of adding it or turning into .mm file, but to which MM format exactly it's not that easy to say.
Many .mm file exists and quiet a few are related to graphics content, so it might very well be possible that more than a single way how to convert png to mm exists.
The most likely conversion is though adding PNG images to FreeMind mindmaps (.mm files), which should be doable with the original brainstorming tool (FreeMind) or any tools that support FreeMind's formats.
Updated: January 10, 2024