Server.Mappath converts a virtual path like "~\resources\images\myPicture.jpg" into a "proper" qualified path that the file system on the server running IIS understands - where "~" is the root folder for your website. You can't feed it a URL and expect it to translate that into a file system address.
If you are trying to save a file into your website, then try
fileupload.SaveAs(Path.Combine(Server.MapPath("~/Media"), fileuploadPimage.FileName);
But be aware that it's very common for different users to have the same name for different files. I'd use a DB to store the filename, the user that uploaded it, and a Guid ID column, and save the file to the disk using the Guid value as the actual filename. That way, different users can safely have files with the same name and they don't interfere with each other.