How to Set default thumbnails size & creating multiple thumbnail sizes?

Registers a new image size. This means WordPress will create a copy of the image with the specified dimensions when a new image is uploaded.

To set the default Featured Image (formerly Post Thumbnail) dimensions use: set_post_thumbnail_size(). 



<?php add_image_size( $name, $width, $height, $crop ); ?>

if ( function_exists( 'add_theme_support' ) ) {
   add_theme_support( 'post-thumbnails');
   set_post_thumbnail_size( 200, 200, true );
}

What does above code really mean?

Does it mean all my thumbnails will be 800x600 or smaller? Because even after setting set_post_thumbnail_size get_post_thumbnail_id($post->ID) points to full-sized thumbnails (like 2500x1200). I don't really need that big images...

When do WordPress create (custom) thumbnails?

Set the image size by cropping the image (not showing part of it):
Set the image size by resizing the image proportionally (without distorting it):

add_image_size( 'custom-size', 220, 220, array( 'left', 'top' ) ); // Hard crop left top

When setting a crop position, the first value in the array is the x axis crop position, the second is the y axis crop position.

• x_crop_position accepts 'left' 'center', or 'right'.
• y_crop_position accepts 'top', 'center', or 'bottom'.

By default, these values default to 'center' when using hard crop mode.

if ( function_exists( 'add_theme_support' ) ) {
  add_theme_support( 'post-thumbnails');
  set_post_thumbnail_size( 900, 600, true );
  add_image_size( 'foo', 400, 300, true );
  add_image_size( 'bar', 200, 250, true ); 
}

When are these custom thumbnails created (like image-900x600.jpg / image-400x300.jpg etc.) ? Only during upload process? What if I'm just changing themes to theme based on add_image_size functions with different thumbnail sizes, do I have to reupload all my images to get right sizes? If yes, then using timthumb over add_image_size wasn't that bad idea..