You can not upload files to the hive like that. In this case you would have to upload the image to a Library on your SharePoint site/web

Read more about the [Hive folders here][1] (for 2010 but still very valid if you replace 14 with 15).

Since these folders  are virtual, every single site and web in your SharePoint Farm will have access to, and use the same files from these folders. That is just one of the reasons why you should not upload files from a webpart here!


  [1]: http://www.myfriedmind.com/2012/05/sharepoint-2010-what-is-the-14-hive/