ResourceManager will work if your image is in a resource file. If it is just a file in your project (let's say the root) you can get it using something like this:
System.Reflection.Assembly assembly = System.Reflection.Assembly.GetExecutingAssembly();
System.IO.Stream file = assembly .GetManifestResourceStream("AssemblyName." + channel);
this.pictureBox1.Image = Image.FromStream(file);
Or if you're in WPF:
private ImageSource GetImage(string channel)
{
StreamResourceInfo sri = Application.GetResourceStream(new Uri("/TestApp;component/" + channel, UriKind.Relative));
BitmapImage bmp = new BitmapImage();
bmp.BeginInit();
bmp.StreamSource = sri.Stream;
bmp.EndInit();
return bmp;
}