Hi there,
It seems like those images are set to display at 150×150. Since iOS devices are retina displays, the images should be 300×300 to display at 150×150 in high resolution.
The reason the other images look fine is they have srcset
attributes to display the larger image on certain resolutions. Not sure why they’re doing that while the other ones aren’t.
To fix, you can go to Settings > Media and make your thumbnails 300×300, then regenerate your thumbnails with a plugin like this: https://wordpress.org/plugins/regenerate-thumbnails/
Then go into the Customizer > Layout > Blog section, and choose the thumbnail size for the featured images.
Lastly, use the width and height values to make the images 150×150.
That way the 300×300 images will resize down to 150×150 and look good on retina devices.
Hope this helps!