image_ms := TMemoryStream.Create;
jpg := TJPEGImage.Create;
jpg.Assign(Image1.Picture.Graphic);
Image1.Free;
jpg.CompressionQuality := 80;
jpg.Compress;
jpg.SaveToStream(image_ms);
Stream := TIdMultipartFormDataStream.Create;
Stream.AddFormField('user', user_id);
result := IdHTTP1.Post('https://site.ru/upload.php', Stream);
Stream.Free;
MultipartFormDataStream.AddFile('file', 'C:\test.rar', 'application/octet-stream');
или
MultipartFormDataStream.AddFile('file', 'test.jpg', 'application/octet-stream', image_ms);