MVC4 exception when saving HttpPostedFileBase for the second time - image

Question

in my ASP.NET MVC4 project I need a way to upload a picture that gets saved to a folder on the server. I came up with the controller code below: it basically works, but only for the first time! When calling this method again (with the same TeacherId to set a new image), I get a System.IO.IOException (HResult -2147024864) that tells me, that the file is in use by another process. How can I avoid the locking? And why does it actually work in the first place? I tidy up all images and streams, but it seams I'm somehow missing something important. Thanks you for your help!
[HttpPost]
public ActionResult Upload(HttpPostedFileBase pic, int TeacherId)
{
if (pic.ContentLength > 0) {
var fileName = "Teacher" + TeacherId.ToString() + "tmp.jpg";
var path = Path.Combine(Server.MapPath("~/TempImages"), fileName);
Image i = Image.FromStream(pic.InputStream, true, true);
if (i.Size.Width > 700 || i.Height > 700)
{
Image resizedImage;
resizedImage = ResizeImage(i, new Size { Width = 700, Height = 700 }, true);
i.Dispose();
resizedImage.Save(path,ImageFormat.Jpeg);
resizedImage.Dispose();
}
else
{
i.Dispose();
}
pic.InputStream.Close();
pic.InputStream.Dispose();
TempData["TeacherId"] = TeacherId.ToString();
return RedirectToAction("EditImage", new { TeacherId = TeacherId });
}
return RedirectToAction("UploadImage", new { TeacherId = TeacherId });
}