Whatever it is that serves up user avatars has been randomly(?) failing for the past several days. I've checked this on several different browsers. If you go to the "Users" page of any SE site, you'll find that at least half of the avatars are missing. This applies to both user-supplied images as well as the system-generated geometric icons.
I had this error too, but I found a solution. I only wish I knew why it was happening in the first place.
Here are the steps I followed, with the screenshots.
Clicked on the current connection (should be WiFi or LAN), then clicked properties:
Select IPv4, then click Properties:
Type in new DNS addresses, 188.8.131.52 and 184.108.40.206 are both Google's: