Can you pls disable all plugins once (specially jetpack and floating social share bar) and see if that makes a difference? (the Jetpack photon module can break the thumbnails)
Also pls check if Thumbnail Generation Method chosen in general tab of theme options is WordPress. If not, pls choose WordPress and see if that helps.
If the above does not help, pls post a temp login to the site as a private message here and I can take a look.