I get the same thing with Daily FX. Ctrl + D or "right-click -> add to speed dial" gives you the little icon. You can click the 3 dots on the speed dial and it'll look like it's a full thumbnail, but it's just the little icon still but with a white background that makes it look like a full thumbnail.
If you click the heart icon on the page to bookmark it, you can use the left and right arrows to choose thumbnails that are actually bigger. But again, one of the choices will be the small icon with a white background that makes it look like it's as full thumbnail. Instead, choose one of the other ones like the "Daily FX - Provided by IG" one for example.
On https://www.dailyfx.com/, if you hit ctrl + u to view the source of the page, you'll see:
<link rel="apple-touch-icon" sizes="57x57" href="https://a.c-dn.net/b/3SYEED/touch-icon-iphone.png"/> <link rel="apple-touch-icon" sizes="76x76" href="https://a.c-dn.net/b/1ztAmA/touch-icon-ipad.png"/> <link rel="apple-touch-icon" sizes="120x120" href="https://a.c-dn.net/b/2Aur0e/touch-icon-iphone-retina.png"/> <link rel="apple-touch-icon" sizes="152x152" href="https://a.c-dn.net/b/2ejr9A/touch-icon-ipad-retina.png"/> <link rel="apple-touch-icon" sizes="167x167" href="https://a.c-dn.net/b/05CBNA/touch-icon-ipad-pro.png"/> <link rel="apple-touch-icon" sizes="180x180" href="https://a.c-dn.net/b/22qJu9/touch-icon-iphone-6-plus.png"/> <link rel="icon" sizes="192x192" href="https://a.c-dn.net/b/390g5P/icon-hd.png"/> <link rel="icon" sizes="128x128" href="https://a.c-dn.net/b/267yYn/icon.png"/>None of those are really good pics for a thumbnail.
If you use the heart icon on https://arstechnica.com/ and click the arrow over a few, you'll see a nice orange one that works great as a thumbnail. That's because the site has a decent one.
Long story short, use he heart icon to set a better thumbnail if a good one is available from the site. If there isn't, edit the speed dial and set the image to a pic you download (like this one that I found on https://images.google.com/).
That's all I can see that you can do.