How do I set a specific size for my image?

Question

How do I set a specific size for my image? For example, I want my image size always be "800px" width and "600px" height. When I upload the any images, it should be that size. It should be responsive too. For the upload manager, if I upload the images is too large, I can crop it ....

 <section class="container-wrap">
 <article class="post-wrap">
    <?php

        $args  = array('post_type' => 'journals');
        $query = new WP_Query($args);

        while($query -> have_posts()) : $query -> the_post();

    ?>

    <a href="<?php the_permalink(); ?>" class="post-img">
        <img width="20" height="20"src="<?php echo(types_render_field('artwork', array('raw' => true) )); ?>">
    </a>
    <header>
        <span class="post-date"><?php echo(types_render_field('date', array('format' => 'm.d.Y') )); ?></span>
        <a href="<?php the_permalink(); ?>" class="post-title">
            <h3><?php echo(types_render_field('title', array('raw' => true) )); ?></h3>
        </a>
    </header>

    <?php endwhile; ?>

</article>
</section>

Answer 1

Use Add_image_size in your functions.php

function theme_sizes(){
 add_image_size( 'custom-size', 800, 600);
}

in your code above use something like:

wp_get_attachment_image( 42, 'custom-size' );

Add_image_size adds a new option when you upload an image, which will then crop the image to the dimensions you need, so you would have thumbnail, medium, full size and your custom-size