Changing Gallery images size?

Question

I'm grabbing URLs of my Gallery images:

$gallery = get_post_gallery_images( $post );

foreach($gallery as $thumbnail) {
   echo $thumbnail_slideshow;
}   

The problem is all I'm getting are thumbnails:

image1-150x150.jpg
image2-150x150.jpg
image3-150x150.jpg

And I want to get:

image1-600x100.jpg
image2-600x100.jpg
image3-600x100.jpg

Of course I have my custom image_size defined in functions.php:

add_image_size( 'mysize', 660, 100, true ); 

And I'm wondering if it is possible to do the easy way using get_post_gallery( $post ) or should create some kind of a loop?

Here's a great answer by Otto, unfortunately it doesn't work for me (I'm trying to make it work in single-customposttype.php, it displays nothing, replacing $post->ID with $page->ID or $parent->ID displays EVERYTHING from Media Library ).

UPDATE

Looks like I don't understand something right.

I took a snippet I've seen around in many places, also in Otto's post above:

$gallery_images = new WP_Query(array(
        'post_parent' => $post->ID,
        'post_type' => 'attachment',
        'post_mime_type' => 'image',
        'post_status' => 'inherit',
));

while ( $gallery_images->have_posts() ) : $gallery_images->the_post();
        the_title(); 
        the_content(); 
        the_permalink();
endwhile;

I pasted it into content.php of Twenty Thirteen and guess what? It returns NOTHING, and of course this post HAS gallery and get_post_gallery_images($post) work on it flawlessly. What am I doing wrong?

Of course when I UPLOAD image to the gallery code above works, because the image is uploaded / attached directly to this post. All of my other images are of course "(Unattached)" and this might be a problem...

UPDATE II

Got it, finally. Looks like Gallery images used to be attached to posts in WP pre-3.5, now it doesn't work this way. Here's it well explained. So the question is - is this solution good and is it safe to use it for the future? Of maybe it's possible to play with get_post_gallery_images sizes? To be honest this function would be so much more powerful if it would load all image sizes in multidimensional array...

Answer 1

Use the shortcode_atts_gallery filter to override the size of the images used before making your call to get_post_gallery_images.

add_filter('shortcode_atts_gallery','force_large_images',10,3);
function force_large_images($out, $pairs, $atts) {
  $out['size'] = 'large';
  return $out;
}

Make sure to remove the filter when you have your data, or all your galleries will always use large images.

remove_filter('shortcode_atts_gallery','force_large_images',10,3);

Answer 2

Two Method in here, if you would like to customize default gallery shortcode, you have to re-edit shortcode!. I think just for now, you are only extract gallery image! So, here is my solution

<?php 

$args = array( 'post_mime_type' => 'image', 'post_type' => 'attachment', 'post_parent' => $post->ID, 'order' => 'ASC');
$attachments = get_children($args);

foreach( $attachments as $attachment) { 
    if ( !empty($attachment->guid ) ) {
        $img_url = wp_get_attachment_image_src($attachment->ID, 'full'); //change *full* size with your custom size

?>