Destination stream the converter will write the image bytes into after this callback returns. Replace it with your own writable stream (e.g. a FileStream for disk persistence or a MemoryStream you intend to read afterwards).
When false (default), the converter closes ImageStream after writing — idiomatic for FileStream replacements that should be flushed to disk. Set to true to keep the stream open after conversion completes (typical for a MemoryStream you intend to read yourself); the caller then owns disposal.