Guaranteed Image Sizes

The answer to you question can be found here: https://stackoverflow.com/questions/10722466/how-do-i-force-wordpress-thumbnails-to-stretch-to-fit-if-uploaded-image-has-smal

Add this code to your functions.php to strip the hardcoded width and height of the images that are integrated inside WordPress:

add_filter( 'post_thumbnail_html', 'remove_thumbnail_dimensions', 10 );
add_filter( 'image_send_to_editor', 'remove_thumbnail_dimensions', 10 );

function remove_thumbnail_dimensions( $html ) {
    $html = preg_replace( '/(width|height)=\"\d*\"\s/', "", $html );
    return $html;
}

and you will need to add the following line to your active theme’s style.css to stretch the image:

.attachment img {
    width: 100%; /* scale to width of container, or whatever you want */
}

Just make sure you are using the correct selector and width / height in your CSS.