Featured Images Not Responsive

Hi,
Your issue can be broken down into two parts – 1) Resized and 2) Responsiveness.

1) Themes generally use some specific pre-defined dimensions to display images on web pages. However, you can create your own thumbnail size according to your requirement.

The following article should give you a good idea about how it works.

http://www.wpbeginner.com/wp-tutorials/how-to-create-additional-image-sizes-in-wordpress/

2) Responsiveness: This feature is controlled by CSS Media Queries. By responsiveness we mean elements with ability to adjust its dimension according to the width of device you are using to view your website. If your theme supports responsive layout it should take care of this automatically unless your images have inline width and height attributes. In order to accomplish the responsive behavior you need to make sure no hard coded dimension gets gendered with images. For this add the following code in your theme’s functions.php file:

function remove_image_size_attributes( $html ) {
    return preg_replace( '/(width|height)="\d*"/', '', $html );
}
 
// Remove image size attributes from post thumbnails
add_filter( 'post_thumbnail_html', 'remove_image_size_attributes' );
 
// Remove image size attributes from images added to a WordPress post
add_filter( 'image_send_to_editor', 'remove_image_size_attributes' );

Let me know if this helps you.