Moodle 3.2.1 using lambda theme does this by default (see attachment).
Just had a quick look through the theme folder and there dont seem to be any structural changes to the dashboard page.
I have also just checked Moodle 3.1. There is a picture by default on the dashboard page of the clean theme.
I suspect this came along by default in Moodle 3.0 and if you are using a Moodle > 3.0 then your theme is probably hiding it with css.
