How can I display image metadata?

I’m trying to add a filter to the_content that will find all images in the post and append the custom attachment metadata credit to it.

Here’s the functions I’ve have:

Adds the Credit field to attachment details page: (this works)

function attachment_field_credit( $field, $post ) {
    $field[ 'credit' ] = array(
        'label' => 'Credit',
        'input' => 'text',
        'value' => get_post_meta( $post->ID, 'credit', true ),
    );
    return $field;
}

Saves the Credit field: (this works)

function attachment_field_credit_save( $post, $attachment ) {
    if( isset( $attachment[ 'credit' ] ) )
        update_post_meta( $post[ 'ID' ], 'credit', $attachment[ 'credit' ] );
    return $post;
}

Search the content for all available images: (this works)

function find_images( $content ) {
    return preg_replace_callback( '/(<\s*img[^>]+)(src\s*=\s*"[^"]+")([^>]+>)/i', array( $this, 'attach_image_credit' ), $content );
}

Appends credit metadata to each image: (this doesn’t work)

function attach_image_credit( $images ) {
    global $post;
    $credit = get_post_meta( $post->ID, 'credit', true );
    $replacement = $images[0] . $credit;
    return $replacement;
}

If I replace $credit value with <span>Hello World!</span> the text will be displayed on the page as intended. There must be something wrong with the way I’m trying to get_the_meta value for credit.

UPDATE

If I manually replace:

get_post_meta( $post->ID, 'credit', true );

with:

get_post_meta( 446, 'credit', true );

It works! So all I need to do is to figure out a way to get the attachment ID.

Because the credit data is saved in the attachment’s post meta, not the main post:

$credit = get_post_meta( $post->ID /* Wrong ID! */, 'credit', true );

Instead you need to catch the ID of the inserted image:

function attach_image_credit( $images ) {
    $return = $images[0];

    // Get the image ID from the unique class added by insert to editor: "wp-image-ID"
    if ( preg_match( '/wp-image-([0-9]+)/', $return, $match ) ) {
        if ( $credit = get_post_meta( $match[1] /* Captured image ID */, 'credit', true ) )
            $return .= $credit;
    }

    return $return;
}