protected function _checkImageVersions($identifier, array $versions) {
		if (!isset($this->_imageVersions[$identifier])) {
			throw new RuntimeException(sprintf('No image version config found for identifier "%s"!', $identifier));
		}

		foreach ($versions as $version) {
			if (!isset($this->_imageVersions[$identifier][$version])) {
				throw new RuntimeException(sprintf('Invalid version "%s" for identifier "%s"!', $identifier, $version));
			}
		}
	}