In this case, please find out this file /jnews/class/Image/Image.php then find function setup_image_size and add_image_size. You may edit those function as your own need. Also, we will recommend you to use JNews Child Theme so the changes that you made won’t lost when theme update. After you have done with the changes, you may need to regenerate all of your images. You can use Regenerate Thumbnails plugin.