YouTube video thumbnails not getting loaded
I had a similar issue with the icons thumbnails that shows a blue box with a question mark in it. I'm running a Mac Pro mid-2010 running High Sierra. 

I finally got it to work after setting the DNS setting to Google DNS.
Apple > System Preferences > Network > Advanced > DNS Tab
And add Google's DNS servers
Click the plus (+) button below DNS Servers table and insert
8.8.8.8
and
8.8.4.4

Click okay and close Safari/Chrome and hopefully it'll be fixed.