This is not a SE issue, but either a gravatar.com problem, some faulty caching proxy at your internet provider, or your own browser messing up.
So: test in another browser, or hit Ctrl-F5 or Ctrl-Shift-R / Command-Shift-R, or hold down Shift when clicking the refresh button, to tell most browsers to reload unconditionally. You might even need to clear all cache and restart your browser.
I can see yours just fine, at http://www.gravatar.com/avatar/c82255633259358535c704fa2ac05e93?s=32&d=identicon&r=PG, and so do Web-Sniffer and Imgur, both from a totally different location in the world.
As for the cause:
My bet is it's your own browser that has somehow cached a broken image. The browser doesn't know the response was faulty (if it would, it should not cache it). And it shows the question mark (or some other "broken image" icon) not because it cannot load the image URL, but because it cannot render the (cached) response as an image.
Peeking into the cache might even show you some other content type, like HTML rather than binary image data;
<img src="https://stackoverflow.com/about"> would also show to indicate a rendering problem.
Gravatar tells browsers to only cache for 5 minutes, but browsers conditionally check for updates when reloading a page, also when a browser has somehow cached a broken image. For default (generated) images, Gravatar returns
Last-Modified: Wed, 11 Jan 1984 08:00:00 GMT. A conditional reload then looks like:
GET /avatar/c82255633259358535c704fa2ac05e93?s=32&d=identicon&r=PG HTTP/1.1
If-Modified-Since: Wed, 11 Jan 1984 08:00:00 GMT
...for which Gravatar responds:
HTTP/1.1 304 Not Modified
This tells your browser its cache is just fine.
The same can happen for non-default avatars, though those (used to) have their Last Modified date set to the date one first subscribed. When not changing the image at Gravatar, the same
HTTP/1.1 304 Not Modified will surely be returned then. But changing it might nowadays change the Last Modified date, and then tell your browser that a new image is available, silently fixing the faulty cache.