replacing all images with a placeholder image in .NET

I'm using a HTTPListener to receive requests and I'm currently trying to replace all the image tags from the response with a placeholder image before writing to ResponseStream. Unfortunately the image doesn't get displayed but instead I see the alt message, even though the image source is correct. I even used the "copy full path" option from Visual Studio to make sure it is (and then changed all the backslashes for forward slashes).

when I copy the image source and open it in a new tab it will open the correct image, so the image src is definitely correct

this is how I filter my HTML string and replace it with the placeholder:

        private string filterContentPlaceholder(string html) {
            Regex imgFilter = new Regex(@"</?img((\s+\w+(\s*=\s*(?:"".*?""|\'.*?\'|[^\'"">\s]+))?)+\s*|\s*)/?>");
            string placeholderSrc = @"file://C:/Users/THE_Full_path";
            string result = imgFilter.Replace(html, $"<img src={placeholderSrc} alt='Placeholder image'/>");
            return result;
        }

I then use this code to write to the stream (in which Output is the string from the previous function):

 private void WriteToResponseStream(string Output, HttpListenerResponse Response) {
            try {
                byte[] bOutput = System.Text.Encoding.UTF8.GetBytes(Output);
                Response.ContentType = "text/html";
                Response.ContentLength64 = bOutput.Length;

                Stream OutputStream = Response.OutputStream;
                OutputStream.Write(bOutput, 0, bOutput.Length);
                OutputStream.Close();
            } catch (Exception e) {
                MessageBox.Show(e.Message);
            }
        }

When I load a website the image doens't get shown but the alt-text does and the source shows up correct in the element inspector. Why isn't it showing the image?