Installation HowTo

Hi,
I’m a beginner in WordPress so sorry for this easy question.

I tried to install it as you discribe it.
I added following to functions.php:

if ( function_exists( 'fly_add_image_size' ) ) {
    fly_add_image_size( 'fly_vorschau', 300, 300, true );
    fly_add_image_size( 'home_page_square_2x', 1000, 1000, true );
}

And in my test post I tried a lot of several things. One of the last tries is:
<?php echo fly_get_attachment_image_src( get_post_thumbnail_id(1277), 'fly_vorschau', true ); ?>

I have this entry in the Backend:
Fly Images Directory
/wp-content/uploads/fly-images
Writeable!

If I open the post in Preview, the lines will be deleted but nothing happens with images. No link and no image generated on the webspace.

What am I doing wrong?

Thanks in advance.