Resize Image While Uploading

i am using file upload control for uploading images. in that iam checking the condition,if Image.Width > 250 || Image.Height > 400 then i am resizing the image. but it is giving the error "The SaveAs method is configured to require a rooted path, and the path 'ProductImages/roman_sandals.jpg' is not rooted."

ProductImages is folder where i am saving image. Can anyone find why this is giving error,my code is

string strBigServerPath = AppHardcodeValue.productImgPath;
string strFileName = "";
if (prodImg.HasFile)
{
strFileName = prodImg.PostedFile.FileName;
string uniqueNum = Convert.ToString(System.Guid.NewGuid());
string shortFileName = System.IO.Path.GetFileName(strFileName);
string Extension = System.IO.Path.GetExtension(prodImg.FileName);
string newFileName = shortFileName;
prodImg.SaveAs(Server.MapPath(strBigServerPath + newFileName));
using (System.Drawing.Image Img =
System.Drawing.Image.FromFile(Server.MapPath(strBigServerPath) + newFileName))
{
if (Img.Width > 250 || Img.Height > 400)
{...........................