You need to use the **CSOM** (Client Object Model) To Download your image. 

Here you use `WebClient` with this, you loose your sharePoint context, and so your are not connected to sharePoint when want to download image by using :  `byte[] bytes = wc.DownloadData(siteurl + fileURL);`  
Try to use methode of the [SPFile][1] object, like [OpenBinaryStream][2] to get the Byte of your file in a Stream.


  [1]: https://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spfile.aspx
  [2]: https://msdn.microsoft.com/en-us/library/microsoft.sharepoint.spfile.openbinarystream.aspx