For some reason, those images "sliders" do not load if the script from stats.wordpress.com is blocked. If you are using a private window with tracking protection, or an add-on that blocks tracking, or a script blocker, that could explain the problem.

About tracking protection (look for the "shield" icon at the left end of the address bar): What happened to Tracking Protection?

About add-ons: Disconnect, Ghostery, NoScript, Privacy Badger, uMatrix, and others may block tracking scripts. I don't see any on your list that I recognize, so this might not be an issue for you. Each such add-on should offer a button or other control to make exceptions as needed to have a site display properly, but you may not always guess that this is the problem...

I use Avast free for protection against virus and I don't know if Avast is the blocker of this scripts. And I don't use private browsing.

Hi pcjordao, Avast has a Firefox add-on and an external filter called Web Shield. While you could try individually disabling those as a test, I'm not sure why Avast would care about that particular server. Are there other sites with missing images that might start to create a more complete picture?

There are other sites with missing images, but like is occasionally, I didn't retrieve those sites and I will try to configure better my Avast. Tanks.