AGO now has Custom Color Themes for the homepage. I switched it to blue and now our custom homepage picture in the banner only shows while logged in as ADMIN. The picture banner is visible when I'm not logged in, which is funny...but when I log in as myself it is not there (it's blank). It's visible for some users but not to others. I cleared cache and tried to view it in Edge and chrome with no luck.
I even re-uploaded the picture and still nothing. I switched the picture to a stock pic and that was visible, so I switched it back to the custom pic and it is not visible.
There is an issue that prevents the cover image from displaying all the time. Specifically, there is a problem with a security token. We've identified the problem and will update AGOL this evening. The fix should address the problem, but please let us know if you have further issue. Thanks for reporting this to us.