Smaller version of images are blurry.

Are your pictures a different aspect ratio than the pixel size you made for thumbnails? Perhaps the thumbnail images are getting distorted because they don’t fit into that pixel size.

No aspect ratio is exactly the same. Perhaps original images are 448*144px, and if I make add_image_size( ‘single-post-image’, 224, 72, true ); (which is 50% of it), it still gets those images blurry.

Here are links for two version of images:
1.) This is the image I made with photoshop 224*72px. This one looks good and sharp. It is the original size. https://www.freespinsfever.co.uk/wp-content/uploads/2017/07/leokuva-png-resized.png
2.) This is the image that was generated by WordPress to 224*72px (original was 448*144px) https://www.freespinsfever.co.uk/wp-content/uploads/2017/07/leokuva-png-2-224×72.png

You can see that other iamge is blurry. Note that if WP resizes it to even smaller, it gets more blurry.

Sorry, the second image is: https://www.freespinsfever.co.uk/wp-content/uploads/2017/07/leokuva-png-2-224×72.png