I currently have this in my functions file, which works as expected:
// Enable Post Thumbnails
add_theme_support('post-thumbnails');
// Custom Thumbnail Sizes
add_image_size( 'square-small-thumb', 80, 80 );
add_image_size( 'square-medium-thumb', 150, 150 );
// Guide image sizes
add_image_size( 'guide-gallerythumb', 142, 85 );
However as it's a large site i'm working on with various post types I don't want my assets folder to be cluttered with too many unused images, therefor I'd like to have some 'global' image sizes but then some image sizes specific to post types, is this possible?
For example, I'd like
add_image_size( 'square-small-thumb', 80, 80 );
To only be generated for the post type 'news'