It's an issue with the Firefox browser. Geoff uses an antique version of FF, but GPS has flagged the same on a later version, and I've just downloaded the latest version to try it out and I get the same. I've re-coded the previous narrow version again, but kept the mobile changes, so mobile users can still see the large image. I'll have to work out what's up with FF on desktop -possibly something to do with pulling the banner image from imgur, but then the narrow version is also hosted on imgur and that seems to work fine with Firefox.
Personally, I think all our users should just get onto Chrome and be done with it... 😉