"Add Media" only shows "Full Size" under Attachment Display Settings

Question

I can't seem to figure out why this dropdown is only showing "Full Size" as the option. I have the default "thumbnail", "medium", and "large" set in Settings, and I've also added a few custom sizes in my functions.php (using this method):

add_image_size( 'single-post-medium', 515 );
add_image_size( 'single-post-small', 250 );

Here's a screenshot of the area I'm speaking of:

It's in the pop up that appears when you click "Add Media" when editing a post.

I'm on Wordpress 4.1.5.

 

EDIT

My issue seemed to be that my thumbnails were not getting created. Images were uploading fine, but no other sizes were getting created. This turned out to be a local environment issue. First thing I did was set uploads folder to 777 recursively. That didn't work. Then I made sure GD was installed and active. It is, I see it when I dump phpInfo. Locally, my thumbnails are still not getting created. Not sure what the issue is there. But as soon as I moved my site to my dev server, the issue was resolved because the thumbnails were getting created.

Answer 1

In order to show new image dimension options in the WordPress admin media library you would need to use the image_size_names_choose filter to assign them a name.

So the code in your functions.php should look something like this:

if ( function_exists( 'add_image_size' ) ) {
    add_image_size( 'single-post-medium', 515 );
    add_image_size( 'single-post-small', 250 );
}

add_filter( 'image_size_names_choose', 'my_custom_sizes' );

function my_custom_sizes( $sizes ) {
    return array_merge( $sizes, array(
        'single-post-medium' => __('Your Medium Size Name'),
        'single-post-small' => __('Your Small Size Name'),
    ) );
}

If they do not show right away, you may need to publish an image first and return to the media library and check again.