src/Model/Gallery.php
private function fillSource($source, $photo): string
    {
        if (! empty($source)) {
            return $this->filterUriInstance($source);
        }

        return (string) $photo;
    }
private function fillSource($source, $photo): string
    {
        if (! empty($source)) {
            return $this->filterUriInstance($source);
        }

        return (string) $photo;
    }
source taken from photo url if not available

@param \Psr\Http\Message\UriInterface|string $source
@param \Psr\Http\Message\UriInterface|string $photo