[OBI assertion spec](https://github.com/mozilla/openbadges/wiki/Assertions] says "Should be a square (90 x 90) and in PNG format." Though, that's not always guaranteed, since it's just a "should".

Will need to play with this quick & dirty CSS a bit more. Tweaked it to 90px, since 100px was actually a mistake. Not even sure the use of `calc()` is a good idea, really