C# Bitmap only loads when static file address is specified

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C#

Windows

Bitmap

form

I'm trying to create a method which loads a image (png) and stores it in a Bitmap variable to be used later. The code is as follows:

private static Bitmap LoadImage(string subfolderName, string imageName) {
           string fileSpec = string.Format(@".\Images\{0}\{1}.png", subfolderName, imageName);

           Bitmap bitmap = new Bitmap(fileSpec);


           return bitmap;
       }

As you can see its fairly simple, I store the file path in fileSpec and then use that as the parameter for creating the new bitmap. The issue is when I use this is that I get the following error:

"Parameter is not Valid"

The error points to this line:

Bitmap bitmap = new Bitmap(fileSpec);  

I have found that this is error is meant to happen when a file is non-existent, but the file is there and when I use a static file address such as this:

private static Bitmap LoadImage(string subfolderName, string imageName) {

            string st = @".\Images\Cards\CardBack_Red.png"; //did this to test if it was working in any capacity at all

            Bitmap bitmap = new Bitmap(st);

            return bitmap;
        }

I get no error and image loads fine. Am I missing something really obvious? Help would really be appreciated, I'm getting really frustrated and desperate at this point.

Thanks.