I got the same validation issue and was able to fix it by overriding the image metadata
add_filter( 'amp_post_template_metadata', 'override_schema_metadata', 10, 2 );
function override_schema_metadata( $metadata, $post ) {
$metadata['image']['width'] = ($metadata['image']['width'] * 1.2);
$metadata['image']['height'] = ($metadata['image']['height'] * 1.2);
return $metadata;
}
the multiplier would make sure that the image is in proportion, not that it matters since the display would still be dictated by the <amp-img> attributes.
