Do the blank avatars always show up in the same pattern? Or are they random? What version of NetworkManager are you using?
Edit:
Open your browser’s inspection tool. Click on the “network” tab and then click on the “Image” subtab to filter the list by image files only. After that, try to click on the ... in a forum post. Do you see bunch of new png files being loaded after clicking ...? If the avatars failed to load, there should be an associated HTTP status code.
Here’s a gif (I apologize in advance for the poor gif quality; open the gif in a new tab to see it clearly) to demonstrate what I mean:
It was just a test to check if it might be related to your ISP or their DNS or your home network. With the USB tethering you basically made sure that the issue is on your PC or your account here.