functions.php

<?php

/**
 * AMP Fallback Image
 * If no image is specified, use a site-wide default.
 *
 */
function ea_amp_fallback_image( $data ) {
		if( ! isset( $data['metadata']['image'] ) || empty( $data['metadata']['image'] ) ) {

			$image = wp_get_attachment_image_src( ct_get_option( 'default_post_image_id' ), 'full' );
			if( $image && ! is_wp_error( $image ) && is_array( $image ) ) {

    		$data['metadata']['image'] = array(
					'@type'  => 'ImageObject',
					'url'    => $image[0],
					'width'  => $image[1],
					'height' => $image[2],
				);
			}
		}
    return $data;
}
add_filter( 'amp_post_template_data', 'ea_amp_fallback_image' );