Problems with image size on the server Wordpress

Question

I have a problem and do not know how to solve it 've tried everything and nothing ! I have a theme that uses 3 different images to the highlighted image , I'm using add_image_size to generate these three image sizes , plus the problem is that the whole picture that I upload it generates 3 ​​sizes ! If I post a gallery will be generated 3 different image sizes for each image and it is consuming too my server. I need it to generate only 3 featured picture images. I would like to do this? Could someone help me the one ? Follow my functions the code size to add image:

if ( function_exists( 'add_theme_support' ) ) {
add_theme_support('post-thumbnails');
add_image_size('ultimas-capa', 370, 240, true);
add_image_size('capa-interno', 1020, 300, true);
add_image_size('blog-capa', 780, 300, true);
}
if ( ! isset( $content_width ) ){
$content_width = 800;
}
add_filter('image_size_names_choose', 'curioso_image_sizes');
function curioso_image_sizes($sizes) {

$mythemesizes = array(
    'blog-capa' => __('Imagem Postagem')
);
$sizes = array_merge($sizes, $mythemesizes);

return $sizes;
}