Specify image dimensions

I ran a performance report for my website (using GTmetrix) and I realized the image dimensions are not set, and I missed specifying the height and width attributes. The theme already had some images, but they are not specified by default. How can I specify them and more importantly, where do I specify the attributes height and width?

<div class="thumb">
    <a class="clip-link" data-id="1" title="Hello world!" href="http://example.com/hello-world/">
    <span class="clip">
        <img src="https://example.com/wp-content/themes/beetube/images/nothumb.png" alt="Hello world!" />
        <span class="vertical-align"></span>
    </span>

In your theme’s functions.php file you can add

add_image_size( 'thumb', 600, 350, true );
add_image_size( 'thumbnil', 600, 350, true );
add_image_size( 'medium', 600, 350, true );
add_image_size( 'large', 600, 350, true );
add_image_size( 'post-thumbnail', 600, 350, true );

add_image_size( '{custom-image-type}', 600, 350, true );

For more information have a look at http://codex.wordpress.org/Function_Reference/add_image_size