Responsive scaling of image for the_post_thumbnail() [closed]

This is more of a css issue than an issue with wordpress. Dimensions specified in the html img tag have a specificity of 0, so any css rules will override them. this should work:

.article-feature-image img {
    width:100%;
    max-width:650px;
    height:auto;
}

And for reference, you could use the wp_get_attachment_image_src function to get the url attribute for the featured image and construct your img tag manually without height and width attributes:

<?php $img_url = wp_get_attachment_image_src(get_post_thumbnail_id(),'kev-feature'); ?>
<img src="https://wordpress.stackexchange.com/questions/137211/<?php echo $img_url[0];?>"/>