How to add custom thumbnail sizes to WordPress theme

Following code will allow you to set custom thumbnail sizes to your WordPress theme. Copy and paste it on your functions.php

 
function max_setup()
{
    add_image_size( 'thumb-small', 200, 200, true ); // Hard crop to exact dimensions (crops sides or top and bottom)
    add_image_size( 'thumb-medium', 600, 9999 ); // Crop to 520px width, unlimited height
    add_image_size( 'thumb-large', 720, 340 ); // Soft proprtional crop to max 720px width, max 340px height
}
add_action( 'after_setup_theme', 'max_setup' );