What shows on a News card when the article has no picture?

Your website's logo. When an article has no picture, its card on the News page shows your logo centered in the picture frame, on your theme's background color, so the card lines up with the cards that have pictures. A website with no logo shows its name there instead. The Featured Spotlight row does the same for any card without a picture, and the Event Calendar for an event without one.

To show a real picture instead, open the article in the website editor and add a Custom Share Graphic in its settings. For an external press link, add its Feature Graphic.