Wordpress - New image size version only for the Featured Image file

You can get the featured image src/source by using the following arguments on the core function (where $post, should be called with global $post in front):

wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' );

How-to use it

I wrote a free plugin available on GitHub, called "Dynamic Image Resize".

You can download & use it for free.

Short Code

Place [dynamic_image] in your content. The shortcode has four arguments:

• src Full path to the image in your upload directory or the ID
• width Integer value
• height Integer value
• classes Css classes – separated by a space

… but there's also a:

Template Tag

global $post;
// Use the template tag with ID *OR* full path
dynamic_image_resize( array(
     // The full path to the image in your uploads folder
     'src'     => wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' );
     // OR: the ID
     'src'     => get_post_thumbnail_id( $post->ID )

    ,'width'   => 60
    ,'height'  => 100
    ,'classes' => 'some classes to align and style the image'
) );

Just dump it in your template where you need it and finito.

Note: It's based on an idea/proposal by Konstantin Kovshenin.

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Sidenote:

If you want to skip/disable default image sizes by default, just add 0 as width and height in the admin settings.

Research

The question Filter For Featured Image lead to this answer: How to hook update_post_meta and delete_post_meta?.

Coupled with the a fine global variable ($_wp_additional_image_sizes) revealed here: How to get a list of all the possible thumbnail sizes set within a theme, bring us to the code that catches the "Use as featured image" action click.

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Code

This code is fired at each "Use as featured image" click. It then goes through all Additional Image Sizes and deletes them, keeping the default WordPress ones (thumbnail, medium, large).
^{Test this code throughly before going LIVE.
GPL Code. No warranty granted. Check the comments.}

/**
    DRAWBACK
    - Setting the Featured Image can be done ONLY ONCE
    -- When first setting the FI, all other attachments intermediate sizes will be deleted
    -- If swapping the FI, the first image's remaining intermediate will be deleted, and the second DON'T HAVE any additional intermediates!
    
    TODO: Restoring deleted intermediates
    - this post may have an answer: https://wordpress.stackexchange.com/a/8082/12615
*/

add_action( 'added_post_meta', 'wpse_57369_selective_delete_intermediates', 10, 4 );
add_action( 'updated_post_meta', 'wpse_57369_selective_delete_intermediates', 10, 4 );

/**
 * Catches the "Used as featured image" action
*/
function wpse_57369_selective_delete_intermediates( $meta_id, $post_id, $meta_key, $meta_value )
{
    if ( '_thumbnail_id' == $meta_key )
    {
        global $_wp_additional_image_sizes;

       /**
        * The global holds all additional image sizes and contains this:
        * 
           array(
           ['post-thumbnail'] => array(
               ['width'] => 1000
               ['height'] => 288
               ['crop'] => 1
           )
           ['large-feature'] => array(
               ['width'] => 1000
               ['height'] => 288
               ['crop'] => 1
           )
           ['small-feature'] => array(
               ['width'] => 500
               ['height'] => 300
               ['crop'] =>
           )
        )
       */

        // Populate a new array with single values based on the keys of the global
        $all_sizes = array();
        foreach ( $_wp_additional_image_sizes as $key => $value )
        {
            $all_sizes[] = $key;
        }
                
        // Retrieve all attachments of current post/page/cpt
        $all_attachs = get_children( array(
                'post_parent' => $post_id,
                'post_type' => 'attachment',
                'numberposts' => -1,
                'post_mime_type' => 'image'
            ));
        
        // Determine upload path
        $uploads   = wp_upload_dir();
        $path_pos  = strpos( $uploads['basedir'], 'wp-content/' ); // start position of the string
        $base_path = substr( $uploads['basedir'], 0, $path_pos);  // path before wp-content, e.g., /etc/public_html/

        // Loop through all attachments
        foreach ( $all_attachs as $key => $value )
        {
            // This is the featured image
            if ( $key == $meta_value)
            {
                wpse_57369_delete_files( $key, $all_sizes, $base_path, 'small-feature' );
            }
            // Rest of attached images
            else
            {
                wpse_57369_delete_files( $key, $all_sizes, $base_path, false );
            }
        }
    }
}

/**
 * Delete all custom intermediates files, except when $keep_size is defined
*/
function wpse_57369_delete_files( $ID, $all_sizes, $base_path, $keep_size=false )
{
    foreach ( $all_sizes as $intermediate )
    {
        /* We need to know the image url [0] and if it exists [3] */
        $the_url = wp_get_attachment_image_src( $ID, $intermediate );

        /* If additional image exist, go ahead */
        if( $the_url[3] )
        {
            // Path of the image to be deleted
            $url_pos  = strpos( $the_url[0], 'wp-content/' );
            $url_end  = substr( $the_url[0], $url_pos);

            // Delete all intermediates
            if ( !$keep_size )
            {
                // loga( $ID . ' - ' . $intermediate, 'delete-me');
                unlink( $base_path . $url_end );
            }
            
            // Featured image, Selective delete
            else
            {
                // Delete intermediate
                if ( $intermediate != $keep_size )
                {
                    // loga( $ID . ' - ' . $intermediate, 'delete-me');
                    unlink( $base_path . $url_end );                    
                }
                
                // Keep intermediate, no action needed
                // PROBABLY, RESTORING AN INEXISTENT IMAGE SIZE MUST BE DONE HERE
                else
                {
                    // loga( $ID . ' - ' . $intermediate, 'keep-me');
                }
            } 
        }
    }
}

function loga()
{
    // This is the FireBug FirePHP console call
    // http://www.firephp.org/HQ/Use.html
}

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Result

Folder contents after uploading images, no Feature Image set yet

Folder contents after setting fondo-restauraciones as the Featured Image

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Other Notes

For dealing with ALL extra image sizes (WordPress defaults and custom defined) use:

$all_sizes = get_intermediate_image_sizes();

/**
 * $all_images contains all intermediate image sizes, WordPress default and declared custom sizes:
 * 
    array(
        [0] => 'thumbnail'
        [1] => 'medium'
        [2] => 'large'
        [3] => 'post-thumbnail'
        [4] => 'large-feature'
        [5] => 'small-feature'
    )
*/