How to specify a rooted path?

I am using <input type="file" id="grpImage" runat="server" /> on a form to let the user upload an image.
In the code-behind, I am trying to save the image on the server to the /uploadedimages directory in my site. (See code below).

The save path is coming out as "c:\uploadedimages\group_1234.jpg" - note the "c:" being tacked on to the front. So I get an error " Could not find a part of the path .. ".

So then I tried setting the image name like this (with the ~):
string imgName = "~/uploadedimages/group_" + strGroupId + "." + fileExt;

But that gives this error:

The SaveAs method is configured to require a rooted path, and the path '~/uploadedimages/group_4294967302.jpg' is not rooted.

What's the correct syntax to use?

if (grpImage.PostedFile.FileName != "")
        {
            string fileNamePath = grpImage.PostedFile.FileName;
            string fileExt = "jpg"; // need logic here to parse the filename to get it

            string imgName = "/uploadedimages/group_" + strGroupId + "." + fileExt;
            grpImage.PostedFile.SaveAs(imgName);
            updatedGroup.GroupImage = "/uploadedimages/skiing_club.jpg";
        }

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