Mural produces rather big PNG, as it's an alternative to PDF documents, and it tries to capture high-resolution data. For example the image I've uploaded is 2,680 × 4,845. That's 12 984 600 pixels, slightly greater than the 12.5 millions of pixel current limit.

MediaWiki uploads it, stores it but can't generate thumbnail.

URL: https://agora.nasqueron.org/File:ServPulse_user_stories_-_Persona_-_Nasqueron_member.png
Error message: Error creating thumbnail: File with dimensions greater than 12.5 MP

Note I was hesitating between upload each persona separately or one file with the 3. That consolidated file would have been 60 MP, not just "slightly over 12.5 MP".

According MediaWiki documentation, the goal is to avoid to be out of memory at thumbnail creation. See https://www.mediawiki.org/wiki/Manual:$wgMaxImageArea