Image quality of thumbs generated issue

Just recently bought the theme and looks good so far but the featured image quality of the thumbs make my site look horrible. When you click through to the post the images are more crisp. I’m not sure what it is. When I go back to my old theme the featured images look fine.

I’ve already used the regenerate thumbs plugin, cleared my browser and checked, no change.
I also tried adding this to my functions.php
add_filter('jpeg_quality', function($arg){return 100;});

Still, no luck. Images are important to my website and if I can’t get them to display more crisp then it makes this theme unusable for me.

Thanks,
Michael

I should add that my original images are bigger than the thumbs so it shouldn’t be a issue of stretching a smaller image to fit a bigger space.