Unless otherwise indicated, the code snippets you see below should be placed into your theme’s functions.php file.
Here is the code needed to add new featured image sizes to your site:
<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.
//* Add new featured image sizes
add_image_size( 'home-bottom', 150, 100, TRUE );
add_image_size( 'home-top', 400, 200, TRUE );