It essentially allows you to upload a "default" header image into the theme and then it replaces the geometric shapes. There is no other way due to how the dashboard widget was created. There is no renderer that can be overridden. The brightest of the bright in the moodleverse looked into this awhile back and there are workarounds but no proper solution could be found.
There is also a trick you can do with templates but it seemed to be involved.
OK, thank you!
Do you mean the 'myoverview' block Peter? And M3.4, 3.5 or 3.6?
Hi Gareth, sorry haven't seen your question until now. Yes, that's what I mean.