Hi, now I’m using WordPress 3.8, but when I use WordPress 3.7.1 it back to me the same error
in the function.php I have this code
grayscale_add_image_size('thumb_actualidad', 500, 500, true, true);
and for print this images on theme I’m use this:
<?php the_post_thumbnail( 'thumb_actualidad-gray' ); ?>
thanks!