Actually, it's fairly obvious:

string relativePath = "~/Uploads";
string absolutePath = Server.MapPath(relativePath);
string fileName = fuImage.FileName;
string fullFilePath = string.Format("{0}/{1}",absolutePath,relativePath);
fuImage.SaveAs(fullFilePath);

Why?
Assume your website root folder is "C:\Websites\Gharuboy".
So

relativePath = "~/Uploads"
absolutePath = "C:\Websites\Gharuboy\Uploads"
fullFilePath = "C:\Websites\Gharuboy\Uploads/~/Uploads"

And

fuImage.SaveAs(fullFilePath);

Will try to save as that file...
No. I think what you want is this:

string fullFilePath = string.Format("{0}/{1}",absolutePath,fileName);