I don't know why, but Wordpress refuses to create thumbnails for images >3 megapixels
The solution to reenable this feature:
1. Open the admin-functions.php file in the /wp-admin folder.
2. look for this:
$max = apply_filters( √É¬¢√¢‚Äö¬¨√ã≈ìwp_thumbnail_creation_size_limit', 3 * 1024 * 1024, $attachment_id, $file );
3. Change the 3 * 1024 * 1024 part to something you think is more appropriate (9*1024*1024 would be 9 megapixels)
4 Save the admin-functions.php file
From now on wordpress will create thumbnails for larger pictures too