Different thumbnail size than actual picture in post

This is quite simple to implement, using core post-thumbnails functionality.

First, you need to add Theme support for the feature, via functions.php:

<?php
function wpse54920_setup_theme() {
    // Support post thumbnails
    add_theme_support( 'post-thumbnails' );
}
add_action( 'after_setup_theme', 'wpse54920_setup_theme' );
?>

Next, you need to define your custom image sizes, using add_image_size(). Let’s say you want the following custom image sizes:

archive-index-featured, 150px x 100px, hard-cropped
post-header-featured, 250px wide, box-resized

Define them as follows (building on our code above):

<?php
function wpse54920_setup_theme() {

    // Support post thumbnails
    add_theme_support( 'post-thumbnails' );

    // Add Archive Index Featured image size
    add_image_size( 'archive-index-featured', 150, 100, true );

    // Add Post Header Featured image size
    add_image_size( 'post-header-featured', 250, 9999, false );
}
add_action( 'after_setup_theme', 'wpse54920_setup_theme' );
?>

Now, you simply need to output the appropriate image size in the appropriate template file, or via the appropriate conditional. For simplicity, I’ll use the latter approach:

<?php
if ( has_post_thumbnail() ) {
    // Determine image size based on context
    $featured_image_size = ( is_single || is_page() ? 'post-header-featured' : 'archive-index-featured' );
    // Output the post featured image
    the_post_thumbnail( $featured_image_size );
}
?>

Alternately, you could simply call the_post_thumbnail( 'post-header-featured' ) in single.php and/or page.php, and call the_post_thumbnail( 'archive-index-featured' ) in other template files.

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