Remove Inline Width and Height from Images

Add the following to your functions.php file:
 

<?php //remove inline width and height added to images
	add_filter( 'post_thumbnail_html', 'remove_thumbnail_dimensions', 10 );
	add_filter( 'image_send_to_editor', 'remove_thumbnail_dimensions', 10 );
	// Removes attached image sizes as well
	add_filter( 'the_content', 'remove_thumbnail_dimensions', 10 );
	function remove_thumbnail_dimensions( $html ) {
    		$html = preg_replace( '/(width|height)=\"\d*\"\s/', "", $html );
    		return $html;
	} ?>

Source: https://snipt.net/dnnsldr/remove-image-height-and-width-for-responsive-design-in-wp/