Currently, it seems not possible to directly include a .png in a dashboard. This (most probably) possible via providing a .md wherein the png file could be referenced. I think, however, this is an unnecessary detour and i am not even sure whether within a .md one could use generic references (e.g. with *_some_name.png). As pngs automatically pop up in the files browser, it would be nice to be able to include them within dashboards.