for naming the image size, you can call it chicken-nuggets if you want. It’s not related to anything, other than needing to know the name to call the image. Anyway, here’s my thumbnail registration code for example from my functions.php:
// THIS IS THUMBNAIL SUPPORT
if ( function_exists( 'add_theme_support' ) ) { // Added in 2.9
add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 75, 75, true ); // default post thumbnails
add_image_size( 'index-post-thumb', 75, 75 );
add_image_size( 'single-post-thumbnail', 150, 150, true ); // Permalink thumbnail size
add_image_size( 'title-image', 50, 27 ); // Title mini photo
add_image_size( 'shop-single-image', 700, 999 ); //Shop Single Product Image
add_image_size( 'member-thumb', 205, 100, true ); // Member Page Image
}
you can see I register thumbnail support, then I define the default thumbnail size, then I make my additional sizes
Now for the conditional its:
<?php if( is_category('the-waking-prince') ) : ?>
<?php the_post_thumbnail('the-waking-prince-thumb'); ?>
<?php elseif( is_category('chicken-nuggets') ) : ?>
<?php the_post_thumbnail('chicken_thumb'); ?>
<?php else : ?>
<?php the_post_thumbnail(); ?>
<?php endif; ?>
so this says, if its the walking prince category, use the waking prince thumbnail, if it’s chicken nuggets, use the chicken thumb thumbnail, otherwise use the default thumbnail
You can add as many elseif lines as you want
now, in your conditional, there is a difference between is_category and in_category
is_category means if the archive page for that category is displayed
in_category means the post is in a certain category
so code accordingly, and I hope I didn’t confuse you too much